Despite you being able to place decals and drawings on the back of the card before (As shown in https://kb.tabletopsimulator.com/game-tools/decal-tool/), it doesn’t work now.
Whenever you place a decal or a drawing on the back of the card, it shows only when you flip that card.
Tested with custom cards as well.
have you tried to toggle “Hide Face Down” ?
I experienced similar issue : https://tabletopsimulator.nolt.io/2004