Please read the documentation of the JSON format: https://json.org/
A string is a sequence of zero or more Unicode characters.
A char is either any Unicode character except "
or \
or control-character,
[...]
or it is \u
followed by four hex-digits.
We are in this last case, \u
followed by four hex-digits, and it represents a Unicode character: Unicode Character 'ZERO WIDTH SPACE' (U+200B). It even has its own Wikipedia page: Zero-width space. And its Stack Overflow question: What's HTML character code 8203?.
Now, there are plenty Unicode characters with special behaviors, and this is one of those, an invisible one among others. So you need to be aware of how Unicode works, and you should sanitize input/output from third-parties API (and from user inputs as well).
Just define the list of characters that you actually want to support, and be sure to strip or filter out all the other ones. For instance, if you desire to support NL and EN, then you could strip what is outside the Latin script in Unicode.
Stripping the U+200B that you're encountering and other undesirable characters may save you from potential surprises like with: