Restricted / lock status not saved in LMS

We have a strange LMS problem with a customer:
We have set the TOC to “Restricted”. So the trainee must work through the slides in a predefined order and all TOC entries are initially displayed with lock symbols, which gradually turn into check marks.
When restarting, the training is set to “always resume”, so the trainee lands on the last slide visited.

We now have the issue that although the position in the trainings is saved successfully, all the lock symbols reappear on restart. So the trainee finds himself on the (checked) slide on which he had left the training, but this slide is surrounded by locked slides.

Is this a known problem? How can we fix it?
Thanks for your help,
Christoph