When a mob converts to another mob, it loses all of its scores.

Exactly same issue as MC-270842 but it doesn't seem to have been fixed properly in this snapshot. (24w36a)

/scoreboard objectives add test dummy
/scoreboard objectives setdisplay sidebar test
/execute summon zombie run scoreboard players set @s test 1

Run the commands above and then convert the zombie to a drowned. You will notice that the score in the sidebar disappears.

/give @s water_bucket