Java Edition 1.15 pre-release 3, vanilla / no mods

Slime chunks do not spawn Slimes.

Tried using commands to kill off everything except player and slimes, repeatedly, but still no slimes spawn at all. Not near a swamp so only testing slime chunks.

Also tried clearing a massive area and setting ground to glowstone, so nothing could spawn except for slimes, waited ages but still nothing.

 Addendum: Slimes do spawn in swamp biomes. Just not anywhere else.