I have found the reason.
I use tax-cash module and the reporting period was not set. Probably because I activated the module or configured it after creating the move.
Solution for that problem is: Check whether you use tax_cash module and have a look at the reporting period.
I solved that, because I had the same problem today, when I added a payment for a customer invoice for December (payment date in Period 2025-12) and the reporting period in the tax line (for tax cash) was set to today() to the new fiscal year-first period 2026-01.
I assume that in
date = Transaction().context.get(
'payment_date', Date.today())
defaults to Date.today(), maybe because the fiscal year differs. It works correctly in the same fiscal year.
From my point of view the payment date should be the correct date for the move. I didn’t dig any deeper yet, because I don’t know which is the indented behavior.
Regards and all the best for the new year,
Jakob