Faulty "assets-" namespace after upgrade from 8.5.5 to 8.6.1

I’ve recently upgraded from Gitlab 8.5.5-ce0 to Gitlab 8.6.1-ce0 and have found some unexpected behaviour.

Namespaces (groups) that start with assets don’t function correctly.
GET requests seem to work, but XHR POSTs all return 405 - Method Not Allowed.

This prevents many actions on projects in the group: adding MR comments, modifying members and so on.

URLs that break when sent a POST are of the form https://local.server/assets<_anything_>/project/...

I see assets is a reserved work and Gitlab rightly prevents creation of an assets group.
Gitlab will allow creation of groups with assets prefix, e.g. assets-design, so I don’t think assets* is intended to be reserved.

Is this a bug or have I perhaps missed some vital upgrade step?
My upgrade method was to step through each patch level releasse from 8.5.5 to 8.6.1 using the official packages on Ubuntu 14.