I am not sure that you have seen my previous post.
No, I indeed missed that post. But it doesn't really matter much as it is unrelated.
Tried Lazarus 4.0rc3 - still the same).
With the exception of unchecking the fpgui package after the selection of BGRABitmap in OPM and just before pressing the install button the result for me in attached picture (same as for 4.0 release btw).