We have on a Ubuntu 18.04LTS server, Drupal 7.63 (latest) with CiviCRM 5.7, PHP 7.1.20, MySQL 5.7.23.
We have a profile form called User Profile that is only used for View/Edit Drupal User Account. All users are synced with CiviCRM so that we can allow our website users to keep their details and preferences up to date and this works perfectly using this profile.
In order to have additional custom fields for a particular type of individual, we have a single additional contact sub-type, based on Individual. The vast majority of our contact records are not of this sub-type.
We've just noticed an issue whereby when an individual updates their profile after they log in on our website, their contact sub-type is also changed on their CiviCRM record along with any other changes that are fine (e.g. their address). The changing of the sub-type is not accessible from this profile form; the only way to change it is to log in as an admin and edit a contact's record.
I can reproduce this with just a save and with no details actually changed at all.
I am going to try and get a more detailed understanding of this by setting up a test site and going through the debugger, but I wondered if anyone else has seen this behaviour? As you can imagine, it's quite serious.