This happens when you put your cursor on a transport or a fleet with a transport and get a tool tip that shows there are legions. It can be any ship or fleet, not just yours. That information then carries over to many tool tips after that. The way to stop it is to put your cursor on any ship or fleet without a transport.
This is due to a bug in the way tool tips get the information to display. It has existed since GalCiv3. I reported the problem in this post in April, 2020. The detail is actually in a file that can be downloaded from Dropbox. The link is still active. The details are in Item #10 on page 3. It shows that this applies to more than legions carried. That document has links that are also still active.
I've seen this in GalCiv4, but I just ignored it since I don't expect it to be fixed.