Hi Barend,
Thank you so much for your quick fix! I can confirm the problem is solved.
I however may have found another bug (just for a single polygon this time, over the same 4000 polygons). The bug was already here before your previous fix.
On the attached images, you can see the result of offsetting the input polygon with a 2.5m distance and the same strategies as above. The output is a multipolygon and it seems to be self intersecting. Is it a bug or is it the expected result? In the first case, I can quickly open a ticket with a sample to reproduce the problem.