If an Armor Stand has the

{Marker:1b}

tag for example:

/summon minecraft:armor_stand ~ ~ ~ {Marker:1b}

If then the player is close to the Armor Stand and approximate to its Y-coordinate and the player is looking in the positive Y-axel (negative in Minecraft, meaning the player is looking up) the Armor Stand stops rendering.

Simplified: Armorstand Markers disappear when looking up (a big problem for map makers)