Hi
I am really confused about how book merging works

Sometimes Calibre merges to first selected sometimes second selected sometimes, order does not matter.
What is the logic of merging order? The reason I want this to be on certain order because sometimes when I merge books one has a cover the other one does not, or I might prefer one cover over other. WHen it is arbitrary I loose cover or other details sometimes
To me it seems like it does not follow the selection order at all even though it says that is what is going to happen.
For instance
-I selected pdf then epub, press "m" or "alt-m" or "shift-alt-m", it says merging to pdf.
-I selected epub then pdf, press "m" or "alt-m" or "shift-alt-m", it says merging to pdf.
As you see the result it the same.
I am using 4.24.2 on Linux x65