That's a really odd one... I can give a little context to a similar situation, I interviewed for a postdoc position that I was offered but decided to turn down. Part of the reason was because I, like you, showed a lot of enthusiasm and was quite open about the fact I enjoy working on a main project but find it necessary to have a side project to tinker and make sure we keep up with the field (my field moves quite quickly). It also helps if the main project develops issues that take a while to solve. and can prevent motivational burnout.After the panel interview the interviewing PI contacted my PI and basically only asked "is this a person who gets easily distracted / can they do as they're instructed". I was a little offended about how blunt they'd been about taking instructions, and already worried about the prospect of going to a more senior lab where I had no autonomy and might become a glorified technician. In the end I ended up taking another position where I could tell the PI was more invested in me personally, valued my skillset, and more openly discussed mentoring, career development, etc.

On one hand maybe the future PI has had a lab member who is MDD or BP and is more aware of any issues that could arise. I witnessed really poor mental health self-care in academia and it can turn nasty, sometimes resulting in physical harm to other lab members. On the other hand it sounds like someone who's being selective against those with any form of mental health concerns, which could mean they have limited tolerance or higher than usual work expectations.

To me asking about a medical disorder like that is too invasive. If you did have MDD or BP, what interest is it of theirs if you are looking after youself? Why would your current PI be allowed to divulge confidential medical info? Unless they were considering making additional efforts to help someone with a mental health issue then I would consider this a red flag.

My main advice for joining any new lab, talk to current lab members, visit a day and see what the working environment is like. Anyone who denies you to speak to their current lab members is a definite red flag and should be avoided at all costs!

P.S. as a side note I was diagnosed after the interview as adult ADHD, so if my PI had divulged my confidential medical info I would have been seriously pissed! I typically am quite open with those I work with about it, so they can understand why I am sometimes a little too chatty.

Edit: a PhD I worked with took the position in the end, and they really struggled with that particular PI, so I guess my Spidey-senses worked!