Tell us what you are trying to do?
Post is posted via a Post form. A category is chosen via a drop-down menu.
That works well.
Then the post is edited via the post edit form.
All fields are filled with the values entered earlier.
Except the Category.
This dropdown remains empty and must be filled in again.
That doesn't sound right. I just double-checked it on my own test site, where it works as expected: if a post has an existing category assigned, then when editing that post with a Toolset Form the existing category will be pre-selected.
Perhaps something went wrong with your form set up.
Are you using the visual form editor or did you switch to the expert mode?
Can you share details of your form set up, in particular for the category input?
That looks just like the default shortcode, and so I would expect it to work.
Would it be possible to get access to your site to check what is happening? (A staging site would be preferal so that I can actually test submitting the forms and if needed add some utility plugins to help with debugging.)
Let me set a private reply to get credentials from you. You may want to create a temporary admin user for me to use that you can later delete. And be sure to have a current backup of your site.
Can you suggest a post to use to test the form with?
I can't account for why, but the problem appears to be specific to that particular form.
If I create a new edit form for Bonusproject posts it works as expected, and the category is pre-selected.
Because you have particular markup in your existing form that you would want to preserve, I checked whether I could copy the markup from your form and paste it in to my form to see if the form would still work correctly, and it did, so the problem seems to be buried somewhere in the hidden form settings rather than in the particular markup you are using in the form editor.
So, might I suggest you create a new form, paste in the markup from your existing form, and use the new form instead?