Main Accounts that require financial dimensions can be posted without financial dimension - even if account structure sets financial dimension as mandatory.
The problem seems to be that only the standard accounts that are posted in a product receipt (e.g. purchase expenditure, un-invoiced) are checked or that not all accounts (e.g. purchase price variance) of the subledger journal or voucher are checked.

If there is an article with item model group standard costs with price variance in purchase order.

If the main accounts for posting types in account structure
- purchase expenditure, un-invoiced
financial dimension is NOT mandatory
- purchase price variance
financial dimension is mandatory

the purchase price variance posting in purchase product receipt is posted without financial dimension even it is mandatory. Also no error or warning occurs.

In my opinion this is an error and/or wrong behaviour of the system.