This issue occurs with the following command

   execute as @s store result score @s om_break run data get entity @s Inventory[2].tag.Damage 1;

while holding a damaged (diamond) pickaxe in the third and any other hotbar slot (when changing the slot number accordingly).

What's supposed to happen:
The damage value of the item is translated/carried over into the "om_break" scoreboard.

What's actually happening:
The Value sometimes gets set to 0, as if the item wasn't found, or was fully repaired.

Steps to recreate:

/scoreboard objectives add om_break dummy
/scoreboard objesctives setdisplay sidebar om_break

Then give yourself a damaged pickaxe:

/give @a minecraft:diamond_pickaxe{Damage:1550s} 1

Then put it in your third inventory slot from the left and run the command (in chat):

execute store result score @s om_break run data get entity @s Inventory[2].tag.Damage 1

The slot numbers of the hotbar start with 0 and go to 8, so this should find the item, and the score should then display 1550, but it shows 0!

Through a bit of testing, the condition for when the Value gets 0 is fulfilled when there are empty inventory slots (here: slot 0 and 1) before the targetted one. Why would this command be dependent from items that are in the other slots?