I have been having this problem for 3 weeks now. I detailed it in the Bambu Studio Bug Reporting Thread
Today is a typical day: the printers showed up for about 3 hours, then lost the connection and asked for the access code for one of the printers. I entered the access code twice and the Studio crashed. When I restored the Studio the printers were no longer there.
The wifi signal on the printers is strong while the signal as shown on the Studio is too weak.
I may try to find some workaround, but I am not the only one having the connection problem so I think it’s up to Bambu Studio to find out why and fix it.