As a matter of fact it is not matter of undead minion: Bug Report on Testlive
As you can see no one was interested in, despite of detailed description of bug… as i suspected before posting this report 
I can only add a workaround for this: You have to level up pet (take all food from him before). Then use only food that give one +14% buff for stat - it’s really better in long term and doesn’t lock buffs.