Companions' Personal section in the Character Sheet erased on removing and re-adding

This is a pretty minor issue, but I've noticed that if I remove and then later re-add companions to my roster, all of their personal character info - like time in party/most powerful killed/most damage taken etc is reset, as though they'd never been in my party before. 

I was wondering if this was intentional or a bug? 

It's not a huge issue, it's just a bit of a shame to not be able to see everyone's stats as the game progresses, and slightly de-incentives switching people out of the party.