Is Pokémon Bank still compatible with ORAS?

I want go go back to my Alpha Sapphire game and catch the legendaries I missed out on. I had already transferred my Gen 4 Legendary Trio to my Pokémon Sun game. Since I caught them in Hoenn, I thought I would be able to transfer them back the Alpha Sapphire so I could catch Dialga, but unfortunately, I couldn't.

Before I reset my game and catch the trio all over again, I would like to know if I am able to store Pokémon I have in my Alpha Sapphire in the bank, reset my game, and still be able to transfer them back into my Alpha Sapphire. This way, I could use an existing team to speed the game up a bit, and hopefully get to catching the trio faster.

Am I able to transfer Pokémon between saves, like this, or is this method a lost cause?

According to the Pokémon Bank page on the official Sun and Moon site, once you use it to transfer a pokémon from gen 6 to gen 7, you won't be able to transfer it back to gen 6. It sounds like this is the problem you encountered.

If you keep your gen 6 pokémon in the bank and they don't touch a gen 7 game, they should be fine to transfer back into your reset Alpha Sapphire.