Done in Chat:

/execute at @p as @s store result score @s dummyObjective run fill ~-1 ~ ~-1 ~1 ~1 ~1 minecraft:air destroy

... does not store the result (some blocks are destroyed), but

/execute at @p as @s store result score @s dummyObjective run fill ~-1 ~ ~-1 ~1 ~1 ~1 minecraft:dirt destroy

...does.

And done in a function:
execute as @a at @s store result score @s dummyObjective run fill ~-1 ~ ~-1 ~1 ~1 ~1 minecraft:air destroy

in both case (chat and function), the amount of the with the fill-command destroyed blocks are not stored in the objective (dummyObjective). If you use any other block, but air it works fine and stores the result.

I realy hope this is a bug.