So sometimes when I change energy gear, for example taking off my energy pet and equipping a pet I want to train, I will lose energy even when I shouldn't.
So say I had 98/110 energy with a dapper corgi (giving 8) and my base energy was 100, if I swapped my dapper corgi for the other pet my energy would drop to 90, instead of staying where it was.
It's not just pets either, this has happened with gear too.