Merge fails if you want to merge "tag" into "Tag" and …

… and “tag” has an id < id if Tag.

So, if you look at the taxonomy screen in wp-admin, and you see:

tag
Tag

and you want to merge tag into Tag, it will fail. Nothing happens, nothing is deleted or merged.

Instead, if you look at the screen and see:

Tag
tag

you will succeed at merging tag into Tag. tag will be deleted and articles belonging to tag will fall into Tag.

Make your tests and report back if I’m right.

hint to replicate: to make appear tag before Tag or viceversa, simply create one before the other. tag than Tag or Tag and than tag.

https://wordpress.org/plugins/term-management-tools/