Transparency under Gimp2.10.6

[Ofnuts]

Works for me:

   

To clear possible misunderstandings:

• On the top layer, you can paint everywhere, the image below doesn't act as a selection mask
  
• The top layer applies to the composition of all layers below it, not just to the layer below.
  
• If you want to apply the effect just on the layer below, you use  a layer group.
  

Without a layer group:

   

With a layer group:

   

Otherwise, make a screenshot of your layer stack (and indicate the layer modes by renaming the layers)