I have a configuration using Household Contacts, and Individuals who are linked to Households by 'Member of'/'Household of' Relationships. The Household holds a Membership which flows down to the Individuals. A Membership sign-up or renewal is done by a logged-in Individual and generates a Contribution attached to this Individual, though the associated master Membership is attached to the Household. This is all set up using a Webform.
Since installing CiviCRM 5.20.0, whenever a Membership is updated one or other of the Memberships in this structure vanishes. Previously on CiviCRM 5.19.3 it worked correctly.
If one of the dates on the Household Membership is changed through the CiviCRM Administrative Interface then: - The Household Membership vanishes. - The associated Individual Membership IS updated with the new date, and appears now to be freestanding belonging to the Member.
If the Membership is renewed through the Webform: - The Household Membership is updated correctly and remains. - The Membership attached to the Individual Contact vanishes. - The Contribution which created the Household's Membership remains.
This renders 5.20 unusable as far as we are concerned. I have also raised this as an Issue at https://lab.civicrm.org/dev/membership/issues/21.