So, I was playing Sword, and I brought Silvally to a den. When I Dynamaxed, Multi-Attack turned to Normal-type.
If Silvally Dynamaxes, the move is displayed as Max Strike, but becomes the Max Move of whichever type of memory it is holding when used.
Though it appears to be a Normal type move, it won't be if you have a Memory attached.
Multi-Attack (Bulbapedia)