I can confirm that this symptom seems to be mitigated, but not entirely eliminated, by login attempts taking place while the server gameplay is paused or paused-by-admin. Unpausing during login seems to increase (but not entirely guarantee) this issue.
When this issue crops up, sometimes gameplay immediately after a user login can "seem" normal for several minutes before the session-fatal desync spiral begins.
I can provide logs if someone can explain to me where the log files are.