A couple of thoughts, for what they are worth:
1) It looks as though a similar issue was reported about a year ago.
Here is the thread: https://fmdev.filemaker.com/message/84673
2) My thought would be to wait a little bit to see if someone has an officially recommended approach for solving this issue. If not, I have a potential workaround that we can try. It is something that I have worked on in order to mitigate the IWP back-button browser issue which I believe is generally considered a known issue. The general idea is: In many cases, we should be able to detect the use of the back button, and when detected, we can run a FileMaker script that will call the ExitApplication script step, thus officially closing the session from the server side. Though I have not tested this extensively, my tests with the workaround have been positive, with failures only associated with the Opera browser.
Note: Truth be told, we don't even know that the workaround will solve the issue that you describe above, as it is still possible that the WPE will crash if a browser tries to connect to a gracefully closed session -- we probably won't know unless/until we try.
My suggestion would be:
Let's see if someone has a proper remedy. It may be that this is a known bug which has been solved with a subsequent release to the FMSA version that you are using -- perhaps someone will post a response that will let us know that is the case. That would be my hope.
Failing the above:
If you are interested in the workaround, feel free to let me know, and I'll explain what would be involved, and you can decide whether or not you would be interested. Though it is rather simple to implement, it is part of a much larger concept, and it has enough ramifications that I think it better not to clutter your situation with the details until such a time as we feel that such an approach might be warranted.
Thanks for the update. I dont believe there will be an "official" response to this or a way forward. I would have found it as I had found the other related issue also (which has no replies). The reason I opened it again was to re-enforce that this is an actual problem that can be replicated.
So I am interested in discussing the potential solution. Lets do it offline for now and we can then post whatever info once we have done the trial fix.
I have sent you my contact details through a PM.