Welcome T J,
Thanks for posting on the Microsoft Q&A forum. I understand it's worrying to be signed out unexpectedly and then find almost nothing in your account activity. I'm happy to explain how that page works.
Is activity reset each month? It isn't a reset. The page covers a rolling window: the Recent activity page shows you when and where you've used your Microsoft account within the last 30 days. Older entries drop off as time passes.
Why you may see only one entry The page doesn't list every sign-in. "We don't show all account activity. You'll usually just see significant events that could impact your account security. If you use the same device in the same location several times in a row, you might only see the first time you signed in." If you mostly sign in from the same console or PC at home, seeing only your latest sign-in can be normal.
To check that your account is safe:
- Go to Recent activity at https://account.live.com/activity and sign in with your Microsoft account. Expand each entry to see its location and how the account was accessed.
- Look for a section called Unusual activity. If there is one, expand each item and choose This was me or This wasn't me. If there's only a Recent activity section, you don't need to confirm any activity.
- If anything looks unfamiliar, go to https://account.microsoft.com/security and select Change password. You can also remove all trusted devices.
- Go to https://account.microsoft.com/devices and check that every device listed belongs to you.
Reference: What is the Recent activity page? – Microsoft Support
Looking forward to your update.