This has been in the for a long time, so I was surprised that I was unable to find an existing report.
When you exit any portal or the world loads when your computer is experiencing a large amount of lag, the game lets you fall. If you fall a short distance, it plays the "trip" sound, and a large distance playd the "fell" sound. This is especially true with a high render distance in the following conditions: exiting the nether to new chunks or to existing chunks with high cpu load (e.g. amplified), loading the end for the first time, entering an end gateway to leave the main island, entering the nether at all, or going really far out in the overworld using a nether portal (i.e. >10,000 blocks) this issue was really bad in 20w49a as every time I'd log in I'd fall into the void (without taking damage), but now in 20w51a it is just when the server can't keep up. It is only for me when I exit a nether portal that isn't within spawn chunks and the chunks aren't already loaded and with the end portal and end gateways.
(This is similar to the effect that occurs when you are on top of a glitched invisible block and you spam jump and then break the block.)
Note: You have to be in survival mode to replicate this.