I tried on your site and it always says “We found 60205 matching post(s) in the cache.” Actually, the cache should be renewed after 60 seconds (by default), showing a message that the database was queried.
If I try with cache completely turned off
https://salescanner.com.au/?redirect_to=random&post_type=product&cache=0&rutpdebug=2
I find less posts: “We found 55695 matching post(s) in the database.”
This means that for some reason your cache doesn’t expire. So the cache is not up to date with the current state of the database.
When I manually add an expiry time, then it doesn’t work with the default of 60, but it does work with other times and the cache expires.
I have no idea why 60 causes the transient to not expire. Maybe there was a bug in a previous version that saved it with an expiry time of 0 (=infinite). You could delete that transient – starting with “chatty_mango_rutp_post_ids” – manually and check then with debugging enabled if the cache will rebuild. Alternatively, you could set the seconds in your link permanently to anything else than 60:
https://salescanner.com.au/?redirect_to=random&post_type=product&cache=59
Or you define the constant if you cannot modify the URL.