When did IIF amount formats change and how can I get it back to old format?
There's been some changes to support php 7.4 which doesn't have money_format() and also some work towards locales. Most likely that's the cause. I'd open a lab ticket.
I'd guess this line needs to be changed to match the other changes that have happened:
https://github.com/civicrm/civicrm-core/blob/33307b5a0da280c63ed0d8fa1f8a3e899a217ad0/CRM/Financial/BAO/ExportFormat/IIF.php#L354
Like this example: https://github.com/civicrm/civicrm-core/pull/19602/files