I've attached 2 videos showcasing the bug, one showing the bug from first person, and the other attempting to show that the Slow Falling effect is active the entire time (although the video lags a bit as its reloading which might look like a cut, but it's not).
I was having difficulty replicating it with just the vanilla datapack installed so I have another one with a bunch of files to create a larger lag spike but nothing that impacts the Slow Falling effect. The Slow Falling icon is present throughout both of the videos which should hopefully be sufficient enough to indicate this.
I can reproduce the bug with my Radeon RX550 in version 1.15.2 and 20w13b.
Can confirm in 1.19.