First, you'll need a canonical tag that uses an ampersand. You can use Character A & Character B on staging, or make a new one by doing the following:
1. Log in with an account with tag wrangling privileges
2. Browse > Tags > New Tag
3. Fill in the desired tag name in "Name," e.g. Character A & Character B
4. Check the "Canonical" checkbox
5. Select a Category
6. Press "Create Tag"
There are certain autocomplete fields where – once you begin typing the tag name – the tag will show up in the autocomplete dropdown list not as Character A & Character B, but instead as Character A &; Character B, with a semicolon after the ampersand (screenshot1). After you select the tag from the dropdown, it will appear correctly on the form, without the semicolon (screenshot2).
The bug doesn't always happen immediately. If it doesn't, try either (a) typing more of the tag name, or (b) selecting the tag from the dropdown, using the × to remove it, and then typing the tag name into the autocomplete field again.
I've had the most luck reproducing it by typing everything up to and including the space after the & sign (e.g. typing "Character A & " for a relationship or "Pete & " for a fandom), but I'm not 100% ready to say that's the cause of the bug.
This bug does not appear to happen on:
It does appear to happen on: