If you have an internet connection that spikes with ping (like mine, reaching 1000ms sometimes) mobs can turn invisible in multiplayer and can still hit you/make sounds.
Reconnecting will make them visible again.
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wobst.michael
migrated
I'm unfortunately unable to test this as I've gotten my connection fixed now.
My suggestion would be to add a F3+[key] bind to resync the client with the server just in case.
Is this still an issue for you in the latest 1.14 development snapshot?