Hi
I tested the trigger you posted and it works ok.
The most likely reason log may show FALSE on that condition is because there have been more than 2.5 goals in the first half at triggering time. First Half Goals markets are 0.5, 1.5 and 2.5. It is feasible to trade each half separately in standard OU markets, but you would need to adjust your expected odds accordingly, and use a condition: selections trigger expression match_period is equal to 1 (or 2) in each half's trigger.
Second reason the log could show FALSE on that condition is the live score data is not available for that event
you could set up a trigger to DELETE EVENT if match_minute is greater than say 15 and score_exists is equal to 0
If it is happening in all markets then check the setting in monitoring options Download Match Score is ticked