I've been plagued by this problem as well... although up until now it's only been an issue when I'm driving in my truck, and as such I'm out of range sooner than I can investigate what's going on.
Thanks to the COVID-19 scare, and now that I'm working from home, this problem has resurfaced. I just happened to find under "Setting > Wifi > Advanced > Hotspot 2.0 (second to last option) > Xfinity Mobile" a button slider for "auto reconnect". This is on a Samsung Note10+, running Android 10, security patch level Mar 1, 2020, Build number QP1A.190711.020... and my software update says I'm on the most current version.
I hope this helps others, so far it's worked for me (going on two hours without an auto-connect attempt)
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