Mobs spawn extremely rarely when updating a map from 18w16a to 1.13-pre2

• When I update my map (created in 18w16a) to the new 1.13 pre-release 2, mobs in the oceans no longer spawn, or spawn extremely rarely! This bug only appears on chunks that has been generated in 18w16a.
• There is also a weird light's bug underwater!
• Mob spawners no longer works too.
• Guardians are not spawning anymore.

Lots of these bugs appears on chunks that has been generated in 18w16a.