Syllabus deletes or moves dates on bulk edit save

use-case: to create future terminated items for a (semi-) regular course in bulk and remove single items (e.g. public holidays).
Since single edit was removed, bulk edit is the only option to make any changes afterwards.
Any update from the bulk edit wrongly makes changes to the dates.

Tested on nightly: https://trunk-mysql.nightly.sakaiproject.org/portal/site/SAK-46385

Observations so far:

This affects all syllabus items as soon as some items have dates assigned. (not related to the bulk add)
For n items (some of them with dates), when x (< n) items are deleted (not from the end but the beginning or in-between), then all following items have their dates edited in each subsequent bulk edit: they move by x.
It is not possible to edit the date of the last item - it will update the (x+1)-th to last instead.

Independent of publish state or if the items were removed manually or via the bulk edit dialog.

Deleted items are not actually removed from the `sakai_syllabus_data` table. But it doesn't seem to matter even if they are.

Current known workaround:

• only delete items AFTER all edits have been made.

• if an edit needs to be made after an item has been deleted: delete all and re-add them, make all edits and only then remove the unwanted items again

Root cause not yet identified. Since I could not find any corresponding service, I would guess shenanigans somewhere in the corresponding jsp.