I was going to ask if this was a bug, but I re-loaded hearthstone just to double check if the issue persisted outside of standard deck construction and discovered the issue was mysteriously resolved. Regardless this WAS the issue:
When making a deck in standard, if you owned a golden version of a card from the original set it was printed in, but hadn’t yet unlocked the golden of it in the core set, you couldn’t use the golden version of that card in your deck.
Reloading the application hours later (not sure if the time frame mattered) resolved the issue. Also, this was of a card that was just crafted and then couldn’t be added into the standard deck. I didn’t test to see if this issue also prevented adding it into a wild deck at the time, and was going to test that when I discovered the issued had resolved.
Just letting you know about this minor bug.