We finally managed to get a series of calls from a *very* helpful person at Insulet, who has opened an investigation.
It would appear that, for some reason, the update (at least, insofar as it has been applied to our PDM) means it now looks for a wifi. If it finds one, it tries to connect. If it knows the password it connects successfully (obviously). If it doesn't, it just keeps trying... until it either (a) finds a wifi network for which it does know the password or (b) gets turned off & back on again. What it doesn't do on failure to connect is fall back to 4G. This is a major change to previous behaviour.
The workaround is to go into the settings and turn wifi connections off entirely, so it now *only* looks for 4G. Since then, we have had correct syncing to cloud all the time. This works fine for us, at present, as there is nowhere we currently go with wifi signal but no 4G. Of course, it may not suit everyone, but then it seems it's only us that's having the issue anyway at present!