Our PRTG Admins also have this exact problem and it has been happening for several months, as well as the problems that others mentioned in this thread about changes reverting or not saving when trying to save. We've had this problem on the Setup\Notification Contacts (changes are not saved), Root\Settings (enable one time maintenance, and then other fields change to old values, such as changing usernames or passwords for Linux), Notification Triggers (don't save), Management page (Cannot apply Multi-Edit to objects because it says a required field isn't filled in, but the field IS filled in), and probably other pages that changes can be made. We keep noticing configuration settings that have been reverted to older configurations, but only discovering that this is the likely reason.
At the least, this is a problem with Chrome Version 71.0.3578.98 (Official Build) (64-bit) and probably a few older chrome versions, since we've been seeing this for a few months now. I haven't been able to replicate it on other browsers, since we primarily use Chrome and it seems that changes need to be made and cached before they become an issue.
The workaround has been to clear the browser cache or using a private browsing session, but I don't consider this a solution because one Admin did this and the problem came back. This problem is basically corrupting our PRTG configuration because we are starting to find inconsistencies throughout our configuration that is likely attributed to this as Admins make changes and other settings are reverted.
Our PRTG is cloud hosted and on version 184.108.40.2065