Villagers are not choosing professions on any servers I've tested on, for the past few Spigot versions. Currently using version git-Spigot-f8e1ae2-a46ef53 (API version 1.14-R0.1-SNAPSHOT)

Placing a world from Single Player onto a server and going to a village, Villagers will stay brown and job-less, and heading to the identical village on SP, they will choose jobs. 
Happened on my main server with plugins, but also 2 test servers with no plugins installed, just Spigot. Also happened with the default world created by my test server.