Jira ClientKey and Jira User Account Id format changes (prefix added)

I’m developing a third-party integration with Jira Cloud and storing user data obtained from Jira - client key (Lifecycle HTTP request payload) and Jira user account id.
I’ve noticed a very weird thing recently - some users of an application are now registered with key and id that have different format:

clientKey : jira:00e8cdc9-a1c6-4f6f-b702-3d4f1b907409 (every user of this particular instance have the prefix, other Jira instances have just a guid - 00e8cdc9-a1c6-4f6f-b702-3d4f1b907409)
jiraUserAccountId : 557058:80af8cfc-e9fd-4692-8dfc-7e58bd73f241 (some other users for this same instance (older ones?) - 5a8c5f3da6ca96323a2d1bd2)

This is not a parsing problem on the application side because the data is written ‘as is’ and various testing instances write guids without prefixes as expected.
This does not seem to cause any issues directly, but users of the above instance experience various bugs across the software. What may have caused these changes on their side? Can that be some GDPR things that were announced recently?

@ovalet, neither of these identifiers have a publicly documented format, and these differences should not impact the functioning of your app. Consider raising bug reports for the issues you’ve seen.

Ok. Are they even configurable?

Configurable by customers? No.