Payments "disappearing"

The following was reported by a law firm using Timeslips Premium 2020
1. Firm receives payment (eg for June Invoice #123 for $500), applies same against open invoice #123, prints and finalizes bill for July #124. The finalized bill for July has a new balance of $1000 consisting entirely of new charges..
2. The following month Firm goes to print next invoice (August, #125), discovers payment of $500 has somehow disappeared and previous balance appears to be unpaid. The unpaid balance now appears to be $1500 - $500 from the June invoice and $1000 from the July invoice.
3. Firm runs data verification, missing payment is suddenly back and appears properly applied. The unpaid prior balance is now $1000, all from July.
4. Once the new invoice is finalized the problem appears to be corrected for THAT SPECIFIC client. The problem is sporadic and appears to move from client to client without discernible pattern. It does not occur every month.

Tech support is stumped.
Anyone else run into this? Any thoughts on a possible cause?