Answer
Check whether a 'Cleanup Application Message Queues' database task is scheduled in your environment — this task is specifically designed to clean up (remove) old Application Messages from the live table after a configured retention period, which is very likely why messages older than a day or two aren't showing up in a normal query anymore. Check the 'Explore Application Message Archive' window to see if those older messages have actually been moved to an archive rather than deleted outright — IFS's Application Message archiving functionality moves processed/older messages there specifically so they remain available for later review, separate from the live Application Messages table the cleanup task keeps trimmed.