After becoming a great power as a migratory tribe, you get a new great family each month

[Leffe7]

1.4.2, no mods.
To replicate: Start the savegame, make peace to get >100 territories and become a great power.
Then each month you get a new clan / great familiy, boosting your stability (+10) but destroying your centralization (-5).
Maybe it has to do with being a migratory tribe?

 

[Leffe7]

Update: The bug appears to happen when a new great family/clan is to be added, but it has the same name as an already existing great family. This can happen multiple times until eventually a new great family is created with a unique name.