The "wblexemeeditformelements" API newly implemented in T184409 specifies a "data" parameter that must look like this:

{
  "representations": { "en": "…" },
  "grammaticalFeatures": []
}

The two fields are currently required. Users of this API must always submit both fields, even if the user only wants to touch one of the fields. Not only that. That other field must contain all the old, unchanged values in order to not make an accidental edit to the field. This makes this API hard to use, and yields the danger of unintentional removals when users run into this issue and aren't aware of all implications.

This is highly relevant because this API currently is the only one that allows to edit these fields. Later, "wbeditentity" might also support editing these fields. However, this will not make the issue raised in this ticket obsolete. It will just raise the same questions again.

It's also not possible to add or remove individual values from either of the fields without resubmitting all the other values that should not be edited. But this is an other question, unrelated to what this ticket is about.