In settings, under Biometrics and Security > Location > Improve Accuracy, do you have Wi-Fi and BT scanning both turned on or off? I have them both turned OFF as I don’t want apps running in the background to override Wi-Fi and BT OFF settings to determine my location independently of my say-so. If you have lots of apps running in the background and they are intermittently using Wi-Fi and BT, this might be a possible source of the ReSound Smart 3D app disconnecting - Google/Samsung’s default setting on installing Android Pie was to turn those two items on and I had to override the suggestion to have them on.
Added Experiment Had Smart 3D running in the background. Launched Google Chrome, Waze, Microsoft Outlook app, The Ultimate ToDo List (paid version a 5-star app!), plus Settings. Played around with Waze map, opened Settings and wandered around and searched. Then used Recent Apps button on Navigation Bar (3 vertical lines |||) to go back to ReSound Smart 3D. As soon as I touched the quickset Noise button, the HA’s suppressed the noise of the running refrigerator. So I didn’t loose any connectivity after 3 or 4 minutes of playing around with other apps.
My phone has a Qualcomm 835 processor, 6 Gb RAM, 64 Gb of internal storage with 38 Gb free. All apps are on internal storage (only photos and videos on SD card), and I’m running the Microsoft Launcher instead of Samsung OneUI or whatever it’s called now. I am currently running the Medium-Power Saving Scheme of Samsung rather than the Optimized power scheme but I haven’t had any connectivity problems with that, either.