To reproduce, type /summon boat (with a trailing space) and move the cursor to the left. See attached screenshot.
MC-124054
Tab-suggestions don't disappear when moving cursor
Resolved
Fixed
0
Confirmed
Minecraft 18w01a, Minecraft 18w06a, Minecraft 1.13
Minecraft 18w30b
Retrieved
Fixed in either 18w30a, or 18w30b.