Chris Webb tells us don’t panic:
Since March 2026, Power BI semantic models have started showing warnings in their Refresh History in the Service. This has scared a few people but in fact all that is happening is that errors which were there all along and which don’t prevent refreshes from completing are now being flagged. Documentation on this feature can be found here but let’s see an example of the type of errors that can cause these warnings.
Click through for that example.