iPhone SE and Resound Bluetooth communication problem

I would agree the phone is most likely culprit. Since OP initially said it occured only when using wifi (or at least that was my take), wifi interference was my first guess. It is definitely a thing and moreso with 2.4 GHz frequency. Yes, some WiFi 4 (n) routers also use 5GHz frequency, but many do not.

Yes but my point is that most WiFi is using both 2.4 and 5 GHz frequencies. Iphones are designed for 2.4 GHz WiFi and Bluetooth to coexist and MFI hearing aids work fine in that environment.