Prevent system to override legal entity in the Ap invoice header once supplier site is changed?

Hey experts,

I need your help and advise, or possibly is there any work around?

Legal entity in the AP invoice header level is sourced from the project, receivables or other modules and for such invoices legal entity is overriden to default legal entity by the system once supplier site is changed and legal entity field is greyed out and there is no possibility to change it back to the legal entity which is sourced from other module. This behaviour happens once invoice is saved and not validated.

This causes an issue in the incorrect accounting where intercompany journal lines are created and there is no work around to fix it.