Not sure, I don’t think I’ve ever done that. I remember reading about some problems with AM and unpacking .arc. Also found some message on Discord where someone was recommending to build arz in AssetManager and arcs with ArchiveTool for his mod, not related to this problem most probably though.
Until someone, for example @mamba or @Serious_Stan chimes in, you could try packing .arc manually with ArchiveTool, command can be found in Exporting Mod Sources - #20 by tqFan
Sometimes typing this command can be a pain in the ass, for example relative paths may not work and you may need to input full paths. I use these (linux command line so different slashes but there should be Windows one in the thread above)
./../ArchiveTool.exe Scripts.arc -add . scripts 9
/C/Program\ Files\ (x86)/Steam/steamapps/common/Grim\ Dawn/ArchiveTool.exe Scripts.arc -add . “C:\Users\MY_USERNAME\Desktop\scripts” 9