I had the exact same issue - event subscriptions in the HSM app for more than half of my sensors were missing.
Here's my theory: HSM setting changes are not materialized into corresponding changes in event subscriptions until the user presses either a "Done with HSM" button, or goes through a disarm-then-arm for smoke/water. And it is a reasonable assumption that most people rarely, if ever, disarm their water/smoke alerts.
The interesting side effect of the above makes me think that the "use every water sensor" toggle actually means "use every water sensor existing today" and does not automatically rebuild event subscriptions on adding new devices or changing drivers.
I guess a workaround would be to schedule daily disarm-then-arm actions at some odd time?
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