Summary
In thold_functions.php (line ~3425), the ALERT>WARNING transition log entry records the wrong threshold bounds:
'threshold_value' => ($warning_breach_up ? $thold_data['time_hi'] : $thold_data['time_low']),
This is the ALERT>WARNING transition path. The log should record time_warning_hi/time_warning_low. Instead it records the alert bounds (time_hi/time_low), so the threshold history shows the wrong trigger value for these transitions.
Fix
Change time_hi to time_warning_hi and time_low to time_warning_low.