I noticed that the first time someone makes a change to a user profile, it's also always changing the field xf_user_privacy.allow_send_personal_conversation for that user from "everyone" to "members"... I think this refers to who can send PMs to that user?
This happens both when I edit someone in the ACP and when a user updates their profile.
I checked the ACP and there's no option to change this field in the UI, so I'm guessing something is happening behind the scenes.
I'm not sure what the correct value of this field should be--if it should be "everyone" or "members", but it should be the same for all these users, not slowly updating one at a time.
If the correct value is "members", then I think this is a bug with the SMF import script not setting it correctly. If the correct value should be "everyone", then something connected to editing a user is causing this bug.
I did wonder if it was related to permissions but I have not changed any of these user's permissions, only the permissions of the user groups they belong to.