I've heard reports of it both here and on other boards.  Seems to be all over the place rather than a systemic issue (i.e. not all taxpayers who have their refunds 100% applied get it).  One preparer said they looked into it and the taxpayer made a ES payment they forgot to tell them about.  Another one included an underpayment penalty (most software defaults this to 4/15) but since the client paid earlier the penalty was lower and the IRS was looking to refund it.  For whatever reason, the IRS seems to think they are sending the taxpayer some money but they don't have an account on file.

The "we've changed your return" letters seem to be much slower than the "we want your bank account number" letters.  If you want to wait six weeks we can start taking bets on what will arrive first, a check from the U.S. Treasury or a letter from the IRS explaining why they're sending money.

Accessing the client's irs.gov account may shed some light on things (if the taxpayer can survive the id.me application).