Found an opportunity to get the secret isu bow in Assassin's Creed Valhalla
JorRaptor posted a video on his YouTube channel showing how to get a secret isu bow called Norden's Arc using a bug.
First you need to go to the lake near Brunson Turret. Find a small island with a rock there.
Next, you need to save and hit the rock (if not broken, load the save and repeat the operation until you break it).
This mythical weapon, which was discovered earlier in the game files, will be hidden there.
However, as it turned out, the developers provided an alternative way to obtain this item.
Darby McDevitt (Narrative Director of Assassin's Creed Valhalla at Ubisoft Montreal) commented on this on Twitter, but without details on how to do it.