This looks to be pulling from the GLPOSTDT field in the PM80100 table. If you post at transaction level without a batch, then the GLPOSTDT in this table is 01-01-1900.
This would have been normal prior to GP 2018 R2 but in GP 2018 R2 we added a feature to be able to Post Through GL at the Transaction Level.
Microsoft Dynamics GP 2018 R2: Transaction Level Post Through General Ledger - Microsoft Dynamics GP Community
In order to make this work with this new feature we would also have to add the GLPOSTDT to the PM80100 which GP never has when posting without a batch.
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