I have a hunch that if you downloaded the latest SceneBuilder (v19) which came out on Oct 7, or if you upgraded SceneBuilder since then, then it is probably starting to save the FXML files with version 19 in them.
You can edit the version number inside the root element of the FXML document -- I suspect you can set it back to 18 without a problem.
On the other hand, perhaps we should be upgrading our projects to use JavaFX 19 instead of 18... shouldn't be too hard since we're using maven, so I bet you could just update the POM.XML file to pull the newer version.
I doubt there are that many big changes between v18 and v19 in any case...