Encumbrance on PO not being relieved

I found that one of three invoices did not relieve encumbrance on a purchase order as expected (three invoices, each for 350, so 1,050 should have been relieved from encumbrance; however, only 700 was relieved).  All invoices posted the debits and credits properly to the GL and I can't actually tell which of the invoices didn't relieve the encumbrance because they are all the same.

Note that this is one example but I have dozens where this is happening - secondary question is how to fix PO encumbrance to be correct without updating the lines.

Has anybody seen this?

Thanks!!