I am curious: 

1) how did you switch back to 2019 and prior? At least in my version of QB Desktop, once you select 2020 W4 the 2019 is no longer selectable. 

2) Are you finding that it is only 'married, filing jointly' employees? If I switch them to 'Single', it withholds the proper amount. 

I have been struggling with this for months and ended up owing tens of thousands of dollars at the end of 2020 due to this issue, since I was paying the amount QB withheld. 

I spent several hours with support yesterday, with no resolution. However, for the time being I am switching all the employees back to 'Single' status so they have something withheld.