The bug

It seems that if a condition in the execute command fails, then the "success" value is not set to 0 when using execute store success.

Setup

/scoreboard objectives add was_successful dummy
/scoreboard objectives add some_score dummy
/scoreboard objectives setdisplay sidebar was_successful

• was_successful will keep track of what /execute store does. We will set the sidebar to show this value.
• some_score is a simple score we can change

How to reproduce

Method 1

We'll set was_successful to 0 so we can observe a change in value.
Lets set some_score to 1, to start with

/scoreboard players set @s was_successful 0
/scoreboard players set @s some_score 1

Now, on running the following command, you'll see was_successful got set to 1, and the text was printed in chat. This is expected.

/execute store success score @s was_successful if entity @s[scores={some_score=1}] run say some_score is 1

Now let's set was_successful to a bogus value (say, 100) so we can observe any changes.
This time, some_score is set to 0, so the above command will not match

/scoreboard players set @s was_successful 100
/scoreboard players set @s some_score 0

Run the above execute store command again.
→ Observe that was_successful did not change. I expect its value to be 0 at this point.

Method 2

Another way to reproduce the bug is described in this comment.