I haven't thought about this particular rotation of Fillet but if you can draw the result with Moi
I believe that you can do it also with Elephant! ;)
Will be maybe not exactly the same animation than Fusion but the result will be the same! ;)

Here an horizontal rotation not yet that you want but just for show Elephant possibilities! ;)
Of course Start End Fillets can be variable if needing... not made completly here :)


And the mystery quasi revealed! :) Hyper simplex : the Pivot point is not yet moving itself!
What a design editor! Thx to Max Smirnov for make such prodigies! Funny challenge...