No responses to my original post, so here it is again with tags.
I got my Galaxy Watch 3 last month and have been enjoying the inactivity reminders feature, quite useful when working long hours at my desk. However, about a week ago, the reminders suddenly stopped appearing, and I cannot get them to re-appear. My activity level/frequency has not changed. I have tried disabling them, restarting, and re-enabling them.
One UI Watch Version: 2.0
Tizen Version: 22.214.171.124
Inactive Time Alerts: On
Schedule: All 7 days, 9AM-11PM
I have it paired with my Galaxy S9.
Galaxy Wearable Version: 126.96.36.1995
Samsung Health Version: 6.17.0.019
Perhaps somewhere either the phone, or the watch, you've disabled notifications for perhaps the system messages? Or, perhaps Samsung Health? Might try searching that out.