When my phone tries to connect to an open wifi, it terminates the mobile connection

Can’t confirm on 2.3-t-20240820427106-dev-FP3, for what it’s worth. Works as intended for me.

Did you use the opportunity of a shutdown to reseat the SIM card, by chance?
And how old is it? SIM cards tend to deteriorate over time and may cause the strangest trouble, also trouble one wouldn’t attribute to them at first.