Are you in a single-player world with cheats disabled, or on a server without operator privilege? Hitting / and then tab should have a result, but if you have started typing a command you don't have access to it will give the behaviour you describe. I can't reproduce this with 1.14.2-pre2 on windows 10 and java 1.8.0_201
This is particularly frustrating because it worked correctly for me between 17w45a and 1.13-pre3 inclusive, during which time I was working on some datapacks for a server I was setting up. I never went live with that server at the time, and having come back to it now to discover it doesn't work even in 1.14.2-pre2 is a real downer.
Using a jump boost of greater than 127 to achieve a negative effect was previously a workaround to MC-2112 in certain cases.
Values greater than 127 are resulting in no effect as of 1.14.2-pre2 and that has broken this workaround.
so... why has a "Won't Fix" bug been "fixed"?