2.4.4 fuyz, religious orders and inheritance

[gimel]

The issue is as follows:

1. Running a Reformed Slavic elective kingdom, firstborn son as chosen heir;
   
2. Second son joins Sons of Perun, which disinherits him, makes it to the grandmaster;
3. First son (genius!) dies, alas, but leaves behind grandson (also genius, yay!);
4. I create a custom Empire, succesion law auto-reverts to gavelkind;
5. Suddenly my second son the grandmaster is the heir apparent, grandson from first son is second in line;
6. Console-killing the second son puts the third son as the heir apparent, grandson still second in line.

I have to ask: is this WAD? Tried recalc_succ in the console, to no avail.
I run a self-built mod, the only thing it changes is de jure setup.