[serial_number ] issue

Hi

I am adding the serial number tag into the subject of our emails.

Recently the serial numbers have not worked correctly. they are not incrementing. They did work until recently.

I have installed a plugin to automatically remove flamingo old messages every 14 days using this plugin: https://he.wordpress.org/plugins/inazo-flamingo-automatically-delete-old-messages/, I am not sure if this is related.

see attached screenshot from flamingo with the serial numbers:

https://www.dropbox.com/scl/fi/92txbmc1rpa17zachjn2v/flaming-serial.png?rlkey=thlxqk8b7m3g5itn627fhqryx&dl=0

Thank you,

Dan