I can totally believe this is a serious issue because of how I've used it to reflect on things and help me overcome trauma from a breakdown I had years ago. I'm *really* careful, I tell it to challenge me, read other sources etc but I've also accepted its encouragement and feedback at times and it can really help. I'm also an expert AI outside of mental health - I know how it works, what it is prone to and that it's categorically not a person or actually empathising with me..
My feeling is if it can contribute to a profound and apparently positive change in my mental health (whilst actively demonstrating hazards and overencouraging me at times) then what is likely going on is I'm experiencing roughly the same thing as these people who are becoming ill - the feedback loop it provides is having a training effect. I'm still going to do it because I and others aronud me see the benefit, but it clearly merits caution. But then so do psychiatric drugs and therapy.