to reproduce:
make score1 and score2 (or similar)
give player test 5 for score1
give player abc 2 for score2
set score1 to the sidebar slot
do /scoreboard players operation cde score1 += * score1
observe, as abc has now gotten a score of 0 int score1
expected behavior: cde becomes the summary of score1, without adding players.
Comments 4
It says source objective, wich implies that this behaviour is not intentional in my example.
Is there a way to reset all scores that are 0, even custom players?
Not without doing so manually.
The entity selector is completely separate from the source objective, and the former will not take the second into account, never has, and that's not a bug.
This was an intentional design choice during the command changes of 1.13.
*means everything on the scoreboard, not everything with a score of this objective, and scoreboard operations cannot work with a "no score", as such, it sets it to 0.