I used to have the problem you’re having with connection only to one hearing aid and not the other about a year ago with my iPhone 7 Plus. But it would happen for a while (like a week), then it would disappear (I could connect with both hearing aids again), then come back randomly after a while (a few weeks). I did not find any solution, but luckily it came and went a few times, but for the most part, I was able connect with both.
And it only happened with my iPhone 7 Plus. I had an iPhone 6 Plus at the time as well, and also an iPad, and this problem never happened with either of these other 2 devices.
But after 3 or 4 random rounds like that over the course of about 3-4 months, the problem went away on its own entirely. I attributed it to maybe new iOS update changes to my iPhone 7 over time that took care of this problem for good.
Then the OPN firmware 6 came along in Q3 2018 and lots of other connectivity issues between my iPhone 7 Plus and the OPN seem to have gone away for good. Now I’d have to say that it’s been fairly rock solid and there has been no more connectivity issues anymore that I’m aware of between Firmware 6.0 on the OPN and iOS 12.1.4 on the iPhone.