At least one group of people who left the workforce during the coronavirus pandemic, and the resulting "great resignation," may not have had much of a choice: mothers.

"Many women were compelled to leave the workforce to care for children who were shifted into virtual schooling or who couldn't attend daycare due to shutdowns," a new report from KPMG, shared exclusively with CNBC's Make It, explained.

The consulting firm's first DEI Progress survey, conducted with the help of Forbes Insights, was put to 300 senior executives, mostly board members, at major U.S. companies across industries including finance, tech and health care.

It found that the "mass exodus" of women and mothers from the workforce is now one of the most pressing diversity-related issues.

The trend is backed up by data: the Los Angeles Times reported that a third of working moms went part-time or quit their jobs since the pandemic began.  

But now that the pandemic is widely regarded as nearing an end, the issue remains, explained Zoe Thompson, the social strategy leader at KPMG U.S.

"Loss of childcare support that started during the pandemic continues to be an issue for many working mothers, who are shouldering the majority of this burden — risking loss of progress on decades of work toward gender equity," she told CNBC's Make It.

Progress slows

Before the pandemic, many businesses were making strides toward gender equity, but their progress has significantly slowed down since, the report found.

"Over the past two and a half years, there has been only a four-percentage-point increase in women among the leadership ranks and a seven-percentage-point increase across the workforce," it said.

Among most of the surveyed companies, women made up just 17% of leadership positions and 25% of total employees as of 2022. Looking ahead to the coming three years, businesses expect both figures to increase by 6%.

"This relatively slow rate of progress is at odds with the fact that women make up 51 percent of the U.S. adult population," the report concluded, adding that it will likely take time before companies return to the pre-pandemic momentum regarding improved gender parity.

As progress on gender equity in the workforce is slowing down, women, especially those in leadership positions are also quitting their jobs at an unprecedented rate, a recent report by LeanIn.org and McKinsey & Company showed.

Diversity, equity and inclusion beyond gender

As well as gender, the KPMG survey also looked at representation of people of color, members of the LGBTQ+ community, veterans, and people with disabilities.

Twenty-eight percent of the overall U.S. workforce are now people of color — a 7 percentage point increase from 2020, according to the report. The amount of people of color in leadership positions rose by 4 percentage points to 19% in the same timeframe.

"Although this progress is encouraging, the absolute numbers remain relatively low," the report said.

Progress has been even slower when it comes to other underrepresented groups, such as those in the LGBTQ+ community or those with disabilities.

The report finds various reasons for this, including that these groups often rely on self-identification from members and that they have not been top of the agenda for businesses, who have instead focused on race and ethnicity.

This could become an issue for the DEI efforts of these firms, the report noted.

"In the long run it is critical that companies acknowledge that, if one group is left behind, it hurts people from all underrepresented groups," it said.  

Overall, 71% of those surveyed said they had taken "many" actions related to their DEI efforts. Some 75% believe strong DEI figures are vital to their business, and almost half of the companies surveyed have added it to their board's agendas.

Despite these efforts, many companies are still finding themselves lagging behind, the survey found.

"Only 12 percent have reported making significant changes in their companies' DEI profiles," it found.

How can companies tackle these issues

Clearly defining terms like "diversity," "equity" and "inclusion," measuring progress through hard data as well as lived experience, and adapting the criteria used based on ever-changing issues affecting underrepresented groups are some of the report's recommendations for companies.

In practice, this means tackling issues quickly and keeping the long term in mind, Thompson explained with the example of mothers leaving the workforce.

"Working women need help immediately, and companies must offer flexibility to all working parents to care for children or elderly parents without professional repercussions," she believes.

"Longer term, companies should partner with other organizations to address the systemic issues that are causing women to step down from their professional responsibilities," Thompson added.

Approaches however have to be unique — what works for one company, underrepresented group, or even individual might not work for others, KPMG U.S. Chief Diversity, Equity and Inclusion Officer Elena Richards explained to CNBC's Make It.

"By no means is there a one-size-fits-all approach. It is important to be transparent about both successes and opportunities, to listen to what our people need, provide the necessary support, re-recruit our talent, reimagine our employee value proposition – all of which ultimately drives accountability," she said.