Steps to reproduce:
(I have just tried it in Survival, I don't know if this doesn't work in creative either)
1.- Place an armor stand.
2.- Put any kind of armor item in your off-hand. (Like a leather helmet or so)
3.- Equip yourself with full armor, so you don't equip yourself with your off-hand item.
4.- Try to equip your armor stand by right clicking it with the armor item in your off-hand.
The armor stand doesn't get equiped by your right-clicking action.
I think this might happen because the game thinks you are trying to equip it on yourself first, so it fails to do both.
Thank you for your report!
We're actually already tracking this issue in MC-83620, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.