Very, very weird post tags issue

I’m having a very strange post tags issue that I haven’t had a problem with before. Granted, I haven’t blogged in about 4 months so I’ve no idea when this may have shown up.

What’s happening is that every time I add a single tag to a post, it gets entered into the database with a term_id of 0, and then is added multiple times (usually 6) to the same blog post, like something is looping the tag over and over again. For example, if I add 4 tags to the original post, after hitting “publish”, all 4 tags multiply by 6, so there are 4 tags, published 6 times, making it 24 tags total on the single post. All 4 tags are entered into the database with a term_id of 0.

I can’t delete them from the post Tags list either – there is no checkbox to select in order to delete them from the list.

To try and solve this issue I’ve:

1. Re-installed 4.3.1
2. Deactivated all plugins
3. Switched themes to 2015
4. Reinstalled my current theme

All with no success. Even with nothing running but the default 2015 theme and naked WordPress, this issue occurs.

Any idea what could be wrong? It appears to happen with every post I make, be it draft or published.