The bug

When /return is executed within an advancement reward function, it will discard all subsequent commands globally, not just the local commands that the function has.

How to reproduce

1. Install the data pack in Attachments
2. Run

   /function mc-266968:a

Results

The following table shows whether each message was expected to be printed or not, and whether it was actually printed or not.

Message	Expected	Actual
a-1	Yes	Yes
a-2	Yes	No
b-1	Yes	Yes
b-2	No	No

As the table suggests, say a-2 in mc-266968:a was not executed because it was discarded by return 0 in mc-266968:b.

Resources

say a-1
advancement grant @s only mc-266968:b
say a-2

advancement revoke @s only mc-266968:b
say b-1
return 0
say b-2

{
  "criteria": {
    "": {
      "trigger": "impossible"
    }
  },
  "rewards": {
    "function": "mc-266968:b"
  }
}

Code analysis

An advancement reward function is executed as an initial function, which has a frame with depth 0, regardless of the current depth. Because /return discards all command queue entries above the depth of the currently executed function, /return executed in an advancement reward function discards all entries, which have a depth >= 0.