Inherited membership is deleted after editing contact when using custom relationship as the inheritance criteria

Tested on 2 production sites and 1 new install sandbox with 3.3.5.

Steps to reproduce
1. Set up custom relationship type relating an individual to an organisation
2. Set up and membership where the above relationship is used to assign membership to contact related in that way.
3. Create an organisation and an individual.
4. Assign the above membership to the organisation and assign the custom relationship between the organisation and the individual
5. Go to individual record (membership is assigned correctly)
6. Edit the individual record and save (the membership is removed)

Also of note is that the relationship is not deleted, just the membership. I have also tested this with 1 of the default relationship types e.g. employer of, and in this case the membership remains when editing the contact so this only happens with custom relationship types.

I happy to do some work on this but I don't really know where to start debugging it.