Session log massed up

I set the log file name to %n_%Y-%m-%d.log, when I open the same session multiple times at the same time, all the sessions' logs are written into a single file.

Reproduce the issue by opening the same session multiple times concurrently with the log filename pattern %n_%Y-%m-%d.log. Trace how session identity and log-file paths are assigned, then verify that simultaneous sessions write to separate files without merging their output.

Written by the indexing model from the issue text.