Hi Andre,
I think this question is more of 'Why did Receivables allow posting a transaction to a closed year?' rather than 'Why did GL allow the transaction to be posted?' Ledger will not allow posting of a closed period/year within Ledger itself (except for the special Post to Prior Year function), but it doesn't control behaviour in other applications.
Currently, Adagio programs (except for JobCost) do not warn during posting that a fiscal period is closed. JobCost is unique in that in has separately maintained 'entry periods' from Ledger. This is why the AR adjustment was able to be posted with an old date.
I seem to recall a discussion elsewhere on the forum regarding closed Ledger periods and posting in sub-ledger batches; if I find it, I will edit this post and copy the link
here.
And to correct this, yes another adjustment for 8.79 needs to be posted in AR with the reverse debits/credits to cancel it to zero.