I’ve written about this in other threads.
I’m having exact same issue and it started with IOS 11.0 and still remains even with 11.1, 11.1.1 and even the 11.2 beta 2.
It happened on my iPhone 7 Plus and my IPhone X.
Despite what others will say, this class of issue was not fixed with the 4.0 firmware.
I’ve changed my HAs and even tried Widex Beyond Fusions and they were affected as well. So it’s not an HA hardware issue.
I’ve written to Apple and they replied to me a few days ago confirming that this is indeed a bug and are investigating it but could not provide an ETa.
I strongly recommend everyone who is having this issue to write to Apple at accessibility@apple.com with complete details on what the issue is, when it started and what you’ve tried. Also mention you’ve heard others have had this issue and be forceful that you want this fixed since it impacts your hearing aids.
If more people report the issue, it will get more attention. I’ve worked at large software companies and issues like these related to accessibility are treated very seriously if there are lots of reports of a bug.