For me this works:

@cwischnewski
I think it doesn't work if you only use the =MyVariable but you need to include the %-signs even though many times in Power Fx you don't use them.
It might also be because I use the Text editor mode or because operator is not equal but contains.
I think the problem might be contains() syntax.
Edit: I actually tried the contains and seems to work.

Also I noticed that it finds the element when I use Test but not in the actual flow. So as @cwischnewski said I think using variables in Power Fx is not supported yet.
Remember to use Text editor toggle on since otherwise the variables might not work with the UI elements.

If you cant figure it out I recommend reconsidering usage of Power Fx and just making new flow without Power Fx(it will most likely give you more problems in the future too since its in preview and might be for a long time)