Blender animation issue when exported to Sketchfab

A very long story, short. (wanted to throw my computer out the window)
I encountered an issue trying to animate objects using Blender - enclosed an example.

I used shape keys to animate the tail wiggle, animations are baked.

The first shark has one material - wiggle ok.
The second shark has multiple materials - wiggle doesn’t work.

Does having multiple materials in one object break the shape keyed animation?
I’ve tried uploading the Blender file and using the Blender exporter - same results.

Hmmm… I am 100% sure having multiple materials doesnrt effect animatio.
I think there is a problem somethere in Blender. Maybe exporting optiona?