The bug

The zero-width space character, rather than being a zero-width space, appears to fall back to GNU Unifont and renders as a control character.

How to reproduce

Paste this into a text field:

a​b

Alternatively, run this command:

/tellraw @p {"text": "a\u200Bb"}

Expected results

The zero-width space would not be visible.

Actual results

It is.

How to fix

Define this as a width of 0 in the space provider file.