This is probably a well known bug but I haven't seen it listed before.

It's quite possible if not easy to have more than 5 sponsors at a shipyard. I've had at least 7 and possibly 8 sponsors previously.

How to replicate: build a shipyard at a colony with available planets nearby. Increase sponsors to 5, then colonize the remaining planets near the shipyard. Newly colonized planets within a certain radius of a shipyard are automatically added to the sponsor list.

Removing a 6th sponsor does not allow you to add it back - only the automatic sponsoring can exceed the intended max.