Aquatic mobs spawn extremely rarely on already generated chunks when updating worlds from 18w16a to pre-release 1 (1.13). Guardians are also affected!
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What happens when you load a pre-18w16 backup in 1.13-pre1 or when you recreate the world in 1.13-pre1 ?

Recreating the world in 1.13-pre1 fix the problem, mobs will spawn again, but everything build in the world in the snapshot 18w16a is gone. If I update my world from 18w16a to 1.13-pre1, only the chunks that I load in 18w16a will have this bug.

Affects 1.13-pre6
Is this still an issue in Minecraft 1.13.1?

No it has been fixed in 1.13.1.