This started happening when I switched to 1.14.4-R0.1-SNAPSHOT. I had a larger plugin I was working on suddenly start preventing the player from moving whenever BlockBreakEvent was cancelled. I then tested it with a minimal plugin that only has one listener function for BlockBreakEvent that cancels the event when called.

Video of the issue

I've attached the minimal reproduction plugin jar to this report.

This issue wasn't present in 1.14.3.