I have been experiencing this issue while playing on a PC. I have tried reinstalling the game and even deleting and restarting my save, but the glitch appears nonetheless.
Any melee weapon Eivor equips disappears during many cutscenes, specifically it is in the most common type of cutscenes, where the dialogue is still skippable (as opposed to fully pre-rendered cutscenes, and dialogues where the player can still control the camera).
I have seen a couple threads mentioning this issue but no sign of a fix from Ubisoft.
Hope it can be resolved soon, the game is super fun but this bug really messes with immersion and enjoyment.