It is likely the store has a lot of interference, which blocks out the signals. This results in you losing the LTE connection and your phone automatically reverts back to the 3G signal.

As 5G becomes more commonplace, I am sure carriers will upgrade their LTE infrastructure to prevent this from happening.

Thanks for the reply. I never never noticed the problem with my IPhone 11 Pro. I think the issue is more with the IPhone 13 Pro. There is a setting called carrier setting in general, about, carrier “Koodo 50”. It may be need to be tweaked by Koodo to accommodate IPhone 13 Pro. When LTE is weak it does switch over to 3G but it takes too long to do so and only if you attempt to use data. The IPhone 11 Pro would switch over seamlessly.

I had same issue on android BYOD samsung 5G phone, switched APN protocol type to ipv4/ipv6 and and never faced this issue.