Not sure if this is a known bug or, if it is in some strange way, expected behavior. Here's an easy way to replicate issue:
Create new document.
Add a vector of any shape
Add a text of any choosing (preferably large font)
Move them close, but don't overlap
Make both black
Compare
Now, make another vector of any shape, make it white and large enough to overlap or cover both black items
Move white shape to very bottom, with two black shapes on top
Select both black shapes, change the opacity to 54% (any % will do, 54% is just what i was working with)
Compare
You will notice, the text is now lighter than the vector where the white box sits underneath, yet both are 54% black.
Converting text to curves does not fix it. I've also attached an example.
John
Affinity Designer 1.4.2 Beta 4, Mac