how to create that bug:

1. sneak on soul soil with boots enchanted on soulspeed (best effect with soulspeed III).
2. stand on the edge of soul soil block.
3. start placing whatever blocks under you.
4. if you did it correctly, you should have speed from soul speed, without soul soil under you.
5. you will have that effect until you will stand on other block, which wasn't place by you when you were bridging.

this bug is for snapshot 20w46a, but you can still do it on version 1.16.4.

I found this bug on my survival world, when i was building a bridge in the nether.

It helped me in faster building my bridge.