When an effect is applied (eg by a tipped arrow) commands can detect the effect with the full duration on mobs, but for players, the duration has already decreased one tick.

Steps to reproduce:

1. Run the following commands:

   /give @s bow
   /give @s tipped_arrow{CustomPotionEffects:[{Id:27,Duration:5,Amplifier:0b}]} 10
   /setblock ~ ~ ~1 repeating_command_block[facing=south]{auto:1b,Command:'execute as @e[nbt={ActiveEffects:[{Id:27}]}] run tellraw @p {"nbt":"ActiveEffects[{Id:27}].Duration","entity":"@s"}'}
   /setblock ~ ~ ~2 chain_command_block[facing=south]{auto:1b,Command:'effect clear @e unluck'}

2. Shoot a mob
3. it returns "5" in chat
4. Shoot a player (eg yourself)
5. it returns "4" in chat, meaning that the duration has already decreased

This is problematic for 2 instances:

1. A duration of 1 tick can be detected on mobs, but not players.
2. If the duration is set to a specific value (eg to the score of a shooter), it will not return the correct value.