iOS: can't sync Firefox iOS (possibly only on simulator?)

[garvankeeley]

This is on Firefox iOS master, and is happening on my simulator and for vphong also in simulator. Not convinced it has anything to do with simulator, that just happens to be where we are seeing it.
The sync keeps stopping with no error output, highly reproducible.
@eoger you are handy with debugging these issues can I get your help to look at it?
I have tried breakpoints in various stages of the sync process, but I haven't seen anything strange just yet.
It is possible this is related to the drop in sync telemetry, but it is too early to say, it is definitely very suspicious.

┆Issue is synchronized with this Jira Task
┆Story Points: 3
┆Epic: a-s consumer issues
┆Sprint: SYNC - end 2020-07-03

[garvankeeley]

I had this reproducing but during bisection it started working. Thus it appears that deleting the app (which clears prefs/keychain) fixes this, which may be helpful to know.