I suspect this is not working because you are not specifying a date in the time comparison. If you don't set it then it will use the current date, but your MainData/Timestamp may be using a different date.
You can change how to generate the comparison date by using the Timestamp, trimming this to just the date, and then using that date string along with the time to compare against.
For example, to get 6:30am on the same day as the timestamp.
parseDateTime(formatDateTime(trimToDays($MainData/Timestamp), 'yyyy-MM-dd') + ' 06:30:00', 'yyyy-MM-dd HH:mm:ss')
https://docs.mendix.com/refguide7/trim-to-date/#5-trimtodaysutc
I hope this helps.