Of course you would have had no delay if you had entered the 1095-a which populated the 8962 on the original tax filing. Sadly due to IRS understaffing, anything requiring human review is taking much longer than it has in the past. Once you return has been corrected for the missing information it still has to go through the regular processing system.
Think of the return process as a long pipeline and along the way your return can be siphoned off two side pools where it will await a human to process the returned. Once you pass that stopped point it's put back into the pipeline where it can be stopped more than once. The more stops it makes the longer it will take for the refund to be issued.
