I saw few other similar issues, but I am not sure this is the same one.

Scenario:

1. Completely new person visits our website
2. Matomo creates records in the table log_visit and log_link_visit_action with new idvisitor say "0x11111111111"
3. The person performs actions A, B and C
4. The person "logs in" as person123
5. Our website calls _paq.push(['setUserId', 'person123']);
   ...at this stage Matomo creates another record in log_link_visit_action and log_visit, but with new idvisitor, say 0x22222222 and with user_id person123
6. User person123 / visitor 0x22222222 performs actions D and E.

Issue: the profile for the user person123 in Matomo only shows actions D and E.
Expectation: the profile for the user person123 should show actions A, B, C, D and E.

In other words, I don't see a reason why Matomo starts seeing the same person as new visitor once she "logged in" (= User ID is set via JS tracking API).