I have a similar scenario where I Set Column values in a couple of tables through relations. Those actions are among others that need to run regardless if the relations are empty or not. If those relations are empty, the entire button disappears.
I perform some actions through a Single Relation, such as a Set Column Action. This action is among other actions in a custom action that sets some values in the current row as well at rows in other tables before executing a Show Detail Screen action. I noticed that the button completely disappears if one of the single relations are empty. If there are no matches in the relations, then there is nothing that to needs to be set, but there is no reason why the rest of the action sequence can’t continue to run. I still need those other actions to occur but the button completely disappears. Short of breaking a single IF branch in the custom action into 3 or 4 separate branches to account for every possible scenario, I see no reason why the entire button needs to be disabled. This was not an issue in classic apps. I did the same thing and the button still worked as expected. This is only an issue introduced with New Apps.