I am able to reproduce this on the demo site as follows:
1) Create a new membership type that is shared by relationship from an Org to an Individual (e.g. use relationship type "Volunteer is")
2) Give an Org a membership of this new type
3) Create a volunteer relationship from this Org to an Individual
4) Verify that the membership is shared to the individual
5) In the Relationship tab, click "more > Disable" for the relationship you set up in #3.
6) Verify that the membership is still shared to the Individual
7) In the relationship tab, click "Edit" beside the now disabled relationship
8) Save without changing anything
9) Verify that the membership is no longer shared
*NOTE: If this is an Employee relationship the membership properly disappears at #6 without requiring steps 7-9.