On occasions when using the Arms warrior rotation the addon will disappear. Sometimes it comes back after killing a mob, other times the main rotation will remain not visible whilst things like Charge will remain visible as a suggested button.
Currently running addon at level 55. I've also noticed alongside this doing this I often get popups saying that I'm experiencing a lot of addon errors. This error message never pops up unless I'm using the Arms Warrior Rotation.

I would need the error message to be able to fix the issue.

I'll see about replicating it and get some screenshots over to you asap.

Message: Interface\AddOns\HeroRotation_Warrior\Arms.lua:364: attempt to index field 'VictoryRush' (a nil value)
Time: Fri Mar 26 12:20:54 2021
Count: 42
Stack: Interface\AddOns\HeroRotation_Warrior\Arms.lua:364: attempt to index field 'VictoryRush' (a nil value)
[string "=[C]"]: ?
[string "@interface\AddOns\HeroRotation_Warrior\Arms.lua"]:364: in function `?'
[string "@interface\AddOns\HeroRotation\Main.lua"]:446: in function <Interface\AddOns\HeroRotation\Main.lua:424>

Locals:

That should be fixed with commit 2bc35b2

You can either update from the github or wait for the next release version.