Steps to replicate:
Be in a bed while other people are on server and not in a bed
Start typing in chat
Unexpected behavior: Within a few words (averaging 15-20 characters), you get disconnected from the server with an error "flying is not supported on this server"
Replication has been pretty consistent for me, but I haven't trial & errored it enough to definitively say 100% reproduction.
Server info: Minecraft 1.14 Pre-Release 2 (1.14 Pre-Release 2/vanilla/snapshot)
Client CPU: Intel Core i7-6700K
Display: GeForce GTX 1070 PCIe running 4.6.0 NVIDIA 417.35 drivers
Please check if that's still an issue for you in the latest 1.14.4 stable release or the latest 1.15 development snapshot.