Hi Nik,
can you switch the mode of the field from dropdown to autocomplete or popup search? Then you could set the value of the field to the workflow id and it will work just fine. I blogged about the same approach here:
https://daniels-notes.de/posts/2022/add-new-choice-field-value-without-leaving-the-page
I have only two other ideas, but I don't know if one will work:
1) When you switch to a different tab, all elements in the current tab are "destroyed" and the new ones are added. It could be that this also applies if a group is collapsed/expanded. Maybe that values of the drop down are retrieved each time, the field is added. Of course it could also be, that the values are retrieved only ones.
2) Did you test, whether you can add the new value as a select option via JavaScript, so that you can set the field to the new value? I doubt that the server will be interested in our changes. If there's any kind on validation on the server, it will use the actual data and not some UI data. This is at least my assumption.
Best regards,
Daniel