Old emails deleting immediately when moved to trash. New emails not deleted immediately.

Following occurs when using Mail app on iPad and iPhone. Also when using web iCloud Mail. I have posted a similar query in the Applications section of this site but so far have not had any responses that advance the matter so hoping I might find something under this category. If the site doesn’t work that way, apologies.

Issue: When I move an older email (dated more than 30 days ago) to trash, it permanently deletes within under 15mins. Newer emails - when moved to trash - are NOT immediately deleted. Those emails stay and are permanently deleted on the 30 day anniversary of their date. Note: they are not deleted 30 days after trashing, but 30 days after the date of the email.

I have the mail behaviour settings on the iPads and iPhone set to remove after one month (all settings relating to mail are identical on all devices). The iCloud website version is set to “Move deleted messages to” bin. I could not find any time period settings on the web version.

All 3 devices are up to date in relation to IOS (although one is older and only security supported). I have no mail rules set up. I have been monitoring this for over 6 weeks and tried turning off and on all the relevant settings for removal and turned the devices on and off to remove any potential glitches.

It seems to me that the deletion is linked to the date of the email not the date of moving to trash - which “seems” to go against everything I am reading. I don't want to keep these things permanently, but when I do a clean up of emails, I want a little bit of leeway in case I delete something I shouldn’t have (which I sometimes do when my eyes glaze over with the clean up!).

Is there another setting that I should be adjusting to stop this happening? Or is it normal behaviour and is the one month removal period linked to the date of the email?