The bug

While the world is starting, functions that are triggered by #load can't find any entities by using the target selector, this didn't happen in Minecraft 1.16.5.

How to reproduce

1. Create a world without any other entities (because the function will print all entities' names).
2. Install the data pack attached to this report.
3. Run the following command to summon an armor stand tagged as "bug_reproduce.dummy" at your location:

   /summon minecraft:armor_stand ~ ~ ~ {Tags:["bug_reproduce.dummy"]}
   

4. Reload data pack(s)

   /reload

   →  The tagged armor stand has been killed, and the server print all entities' names.

   [14:02:27] [Server thread/INFO]: [Server] [bug_reproduce] start
   [14:02:27] [Server thread/INFO]: [Server] Found: bill96012, Armor Stand <end>
   [14:02:27] [Server thread/INFO]: [Server] killed bug_reproduce.dummy
   [14:02:27] [Server thread/INFO]: [Server] [bug_reproduce] end
   

5. Summon tagged armor stand again(step 3), then quit to title.
6. Re-enter the world.
   →  The tagged armor stand is still alive, and the server didn't find any entities to print.

   [14:02:33] [Server thread/INFO]: [Server] [bug_reproduce] start
   [14:02:33] [Server thread/INFO]: [Server] Found: <end>
   [14:02:33] [Server thread/INFO]: [Server] [bug_reproduce] end
   

   →  In Minecraft 1.16.5, it works as step 4 does.

Data pack

#minecraft:load

{
    "values":[
        "bug_reproduce:test1"
    ]
}

bug_reproduce:test1

say [bug_reproduce] start

scoreboard objectives add test dummy

say Found: @e <end>

execute store result score #killed_dummy test run kill @e[tag=bug_reproduce.dummy]
execute if score #killed_dummy test matches 1.. run say killed bug_reproduce.dummy

say [bug_reproduce] end