To-date I have never been billed. That may change, but I think they can’t handle it. Too busy trying to service the idiots that comply.
They always find a way to bill me. The safe harbor is paying in 100% of prior year's tax due or 90% of current year's tax. My income varies by year so I can't get estimates perfect. Just last week I got a $1,500 bill for underpayment of estimated tax for the 2020 tax year. Perhaps you're paying during the course of the year through withholdings?
Incidentally, they still haven't cashed the $35k check I sent them in April for tax due with my 2021 return despite acknowledging that they received it. I guess they don't need the money. :-)