Interesting. I have the same issue with my ReSound Omnias and it happens about once a day. I’m not sure whether it involves “iPhone separation,” streaming, or using my Apple watch to turn the external mics on or off or otherwise remotely control the Smart 3D app on my iPhone. I’ve associated it (perhaps wrongly) with the upgrade to iOS 16.3.1, since I didn’t notice it before that update. After uninstalling the Smart 3D app, unpairing and repairing my HA’s, and reinstalling the Smart 3D app, often just force-killing and relaunching the Smart 3D app is enough to restore connectivity to the disconnected HA (which recently has usually been the right). Otherwise, just as for you, resetting the affected HA fixes the problem. The ReSound Omnia is a MFi hearing device. Other posters in this thread assume that you are an iPhone user. Is that so?
Threads related to my Omnia connectivity problem: iOS 16.3.1 and Connectivity Issues and Resound Omnia 9 battery problem - #5 by jim_lewis. Another user has had similar problems with a number of Omnias and the ReSound One on their iPhone also (see 2nd thread).