The mod depends hard on Data Attributes as mentioned on the CurseForge page (and well, seen by logs when starting without it). Data Attributes should be added to the manifest as a dependency:
https://github.com/CleverNucleus/PlayerEx/blob/287b7ca28d01d13584978a82f502278486d7155e/src/main/resources/fabric.mod.json#L42-L47

I also don't think this belongs here:
https://github.com/CleverNucleus/PlayerEx/blob/287b7ca28d01d13584978a82f502278486d7155e/src/main/resources/fabric.mod.json#L48-L50

Ah well spotted - I'll amend that in the next release. Thanks.

Fixed.