This is unconfirmed for now, one source allegedly said to one media outlet (reputable oppositional one though):

“Such injuries, described by those that saw them, appear from seizures,” the unnamed paramedic told the exiled Novaya Gazeta newspaper.

I mean it could be true but the title makes it sound too certain.

UPD This new Mediazona story is interesting though:

Midnight convoy on the ice road. Live cam footage likely captured Russian authorities transporting Navalny’s body

https://en.zona.media/article/2024/02/18/crossing