I did discover that a few vanilla tweaks datapack was still loaded (i'd loaded a single player save after backing up). I've deactivated them, and now it's behaving. I thought the datapacks wouldn't load after a version change without rerunning the enable command?
So my bad.
I'm seeing this as well in 1.15.1 on Ubuntu. But its not consistent. Sometimes I load the game and its working, sometimes I load and the conduit doesn't kill mobs, and while animated makes no sound.
I'm having this issue in vanilla 1.17.1 on linux.
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