The bug

Marker armor stands cannot find themselves when a value of 0 is used as distance selector argument value.

How to reproduce

1. Summon a non-marker armor stand

   /summon armor_stand ~ ~ ~ {Tags:["non_marker"]}
   

2. Have the armor stand use a distance=0 selector

   /execute as @e[type=armor_stand,tag=non_marker] at @s run say @s[distance=0]
   

3. Summon a marker armor stand

   /summon armor_stand ~ ~ ~ {Marker:1b,Tags:["marker"]}
   

   The armor stand finds itself as expected

4. Have the armor stand use a distance=0 selector

   /execute as @e[type=armor_stand,tag=marker] at @s run say @s[distance=0]
   

   The armor stand does not find itself