Are you adding a tag to your CFM via the Properties console, or it is when the CFM contains the field of type 'Tags'?
If it is the latter, then this seems to be related to a known issue [1] that has been fixed in AEM Service Pack 12.
[1]
The content fragment editor does not work correctly (Its Content Fragment Model contains the field of data type "Tags") when capital case letters are used in tag names when editing tags in the editor (NPR-37601)
(When a content fragment is created from such a model, the tags selected from the tag field do not persist on doing a save & close.)
https://experienceleague.adobe.com/docs/experience-manager-65/release-notes/service-pack/6.5.12.html...