In UE5.5 Epic has changed the default importer to the Interchange FBX importer and it's been causing a lot of issues for folks, with things not working as they should, or blendshape animations getting imported as a completely separate anims. It's bad! Fortunately you can disable it with Interchange.FeatureFlags.Import.FBX 0
.
You can also chuck it into the ConsoleVariables.ini in the Engine/Config
folder of your engine install