In the tick when a marker Armorstand is summoned it seems to have the Hitbox of a normal Armorstand. One tick later the hitbox is corrected.

This can be observed with two repeat commandblocks on top of each other:

• lower block:

  /summon armor_stand ~ ~ ~ {NoGravity:1b,Marker:1b}

• upper block:

  /execute @e[dy=0] ~ ~ ~ /say hi

If you power the upper commandblock first and then the second, everything is as expected and nothing is printed in chat, because the upper command is triggered first in the tick.

If you first power the lower block and then the upper, the chat will be spammed with hi as the Armorstand is summoned and immediately tested for. The correct behaviour would be, that the Armorstand is still not found.

You can also change the upper repeating commandblock to a chain commandblock, wich is always active to observe the bug.

/kill @e[type=armor_stand,r=5]

summon falling_block ~ ~1 ~ {Block:redstone_block,Time:1,Passengers:[{id:falling_block,Block:activator_rail,Time:1},{id:commandblock_minecart,Command:fill ~1 ~-2 ~1 ~3 ~-1 ~1 air},{id:commandblock_minecart,Command:setblock ~2 ~-2 ~1 repeating_command_block 1 replace {Command:summon armor_stand ~ ~ ~ {NoGravity:1b,Marker:1b}}},{id:commandblock_minecart,Command:setblock ~2 ~-1 ~1 repeating_command_block 1 replace {Command:execute @e[dy=0] ~ ~ ~ /say hi}},{id:commandblock_minecart,Command:setblock ~ ~2 ~ command_block 0 replace {Command:fill ~ ~-4 ~ ~ ~ ~ air}},{id:commandblock_minecart,Command:setblock ~ ~1 ~ redstone_block},{id:commandblock_minecart,Command:kill @e[type=commandblock_minecart,r=0]}]}