I can also confirm this bug occurring in 19w08b, as well as 19w08a. Reloading entities seems random, and I have yet to find a consistent way to temporarily reload all missing entities. I've tried allocating more RAM to the server and client, with varying results. Changing the RAM usage of the server, then removing the main world file, generating a new world file, then replacing the old one and loading the server up again seemed to fix it for me, only if for half an hour. or however long I remained within the loaded chunks, but this could also just be coincidence. Armor stands and boats disappear and reappear, as you'd expect from entities, but even furnaces, blast furnaces, and brewing stands can stop functioning altogether. Breaking and replacing them within the chunk does not work. Named entities like pets are not immune and have the potential to disappear.
I don't really know how to reproduce it, it seems to randomly happen after sleeping. Logging in & out turns it back to normal. The rain visual is expected to stop, but instead continues, also allowing Riptide.