Both indicated that their underwriters would likely check up after closing - 30 or 60 days after - to check on income at that point, to see if there were problems, and that this was somewhat normal for them with self-employed folks, but wasn't anything related to c19 (which... I suspect some changes may occur to underwriting and loan processes in the aftermath of c19). They indicated this was just something that happens to 'a moderate portion of self-employed borrowers post-closing'.
I'm not sure what they could actually do to you if they found your income had gone down or away post-closing. I was going to ask in our next call, but ... we're probably putting that whole project on the shelf for now.
(Also, unless you have your savings with the mortgage lender, I don’t think the bank can legally check your balances without a some sort of soft credit pull)
If not maintained above a certain ratio, the lender has the right to come and take over your bank accounts and you have to submit to them to get reimbursed for expenses.
Never heard of it in a residential loan though.