Regular Item purchase Cost is posted into G/L as negative credit

Hello everyone,

once again I have an extremely weird case:
- user are doing a regular purchase of items (POs -> warehouse receipt -> combined invoice)
- item ledger entries, value entries, vendor ledger entries are fine, nothing wrong with them
- but G/L entries for direct inventory cost are sometimes wrong, they were posted as "correction" (instead of increasing inventory G/L account, they "decrease" it by negative amount).
- I cannot replicate the same myself, I have even tried copying purchase document from a posted purchase invoice

Has anyone had anything alike? Or does anyone have any guesses? Even the wildest are welcome...

Alan