Adding a "Reply-To" header to an email filter doesn't appear to work.

I am running Thunderbird 140.7.1esr (64 bit) running on Ubuntu 22.04. I need to filter some emails, and need to filter by the "reply to" email. Reading elsewhere, I followed the instructions, and created a new custom tag to filter (and checked in the advanced part of the settings, and saw that under the mailnews.xxxx there was the "Reply-To" header set up. I then set up a filter to use this new tag (sorry if my terminology is incorrect), and ran it on a mailbox, where I know there are emails with the Reply To email address is present, and not one of the emails was affected by the filter. What am I doing wrong? Many thanks for your help.