I took a command block and typed in a summon command, as I get to NBT I typed things and nothing happaned (its not on the screenshot), so I checked a lot and I did not see any error. I thought that this tag changed on 1.13 but the command block didn't say nothing and the NBT wasn't red.
I am not sure if it's a bug but also if not, fix this because I can't learn in my head any tag...
So you expect the syntax parser to tell you that the following is invalid :
But it's a valid nbt, its syntax is valid. Sure the armor_stand doesn't have such NBT, but the syntax parser doesn't really know that.
Also you don't really need to learn nbt, you can either use the wiki to have the full list, or you could just use /data get entity @e[type=armor_stand,count=1,sort=nearest]