Hydraulic brake lever doesn't return after being released, dependent on weather
I think you should try a bleed.  We could go back and forth all day about what could be causing the problem, but it will all be moot until we have ruled out air in the hydraulic system.
I had a pair of brakes (only one actually) that one day just went totally slack.  Literally fine one moment, and dead the next.  I was convinced there was a leak.  Why else would it go from working to not working so quickly?.  I asked the guy at the LBS how to go go about diagnosing and fixing the leak.  He suggested I bleed first, with out assuming a leak.   I bled the brake...problem solved... no leak after many miles of riding.
The moral of the story here, as it was explained to me by my friendly LBS guy is that when we are having trouble with anything it is advisable to proceed with typical maintenance procedures before we go looking for things that are broken. This will often take care of the problem.  If it doesn't solve our problem, our time is not wasted because the maintenance is something we would want to be doing anyway.