Bug overview description.
Tags that should have already existed are being marked as have being added.

What part(s) of the site page(s) are affected?
Tag History Viewer

What is the expected behavior?
Adding only tags given

What actual behavior is given instead?
Adding tags given and tags that are implicated (such as the new pantherine tag)

Time of incident (if applicable).
6 occasions on Dec 22, 2018.

Can you reproduce the bug every time?
Yes, every tag edit made after tag implication update.

What steps did you take to replicate this bug?
Update any tags to an image from before a tag it already contained was implicated.

Errors or other messages returned (if any).
No errors returned by e621. But a code I wrote gives me a security warning every time extra tags are added.

This is "intended" behavior. The website doesn't create explicit history actions for aliases and implications when they are created. So when the post is next edited, it compares the tags with those in the tag history and considers them as having been added in the next edit.

My guess is that this was done in order to save time, as creating hundreds of thousands of history items when adding aliases or implications tends to become impractical.