Armor stands have the ability to be set to invisible. This invisibility works in all gamemodes apart from one. In SPECTATOR mode the player will see aprox 50% opacity version of the armor stand, despite it having the invisibility tag set to true.

Steps to reproduce;
1) Spawn armor stand that has invisibility enabled
2) switch to SPECTATOR mode
3) observe that it is not invisible

Reasons why this is a really dumb design choice:

• Holograms are basically impossible to use practically along with spectator mode
• There is no logical reason why a player in spectator mode should need to see invisible armor stands because they are used in many "behind the scenes" areas of adventure/custom maps.

If this isn't fixed then Spectator mode becomes a large enough inconvenience to prevent its use, especially on servers or other vanilla custom game modes that utilize holograms or other floating items with armor stands.