Hi,
thanks for reporting this!
It is indeed a bug.
I just separated the nonlinear constraints at the initial and path stage a week ago, and unfortunately introduced it then, see Separate nonlinear constraints into path and initial stage constraints by FreyJo · Pull Request #994 · acados/acados · GitHub
I pushed a fix and will merge it soon here: Fix bugs with slack dimensions Python interface by FreyJo · Pull Request #1001 · acados/acados · GitHub