When you fly into a ladder with an elytra, it will treat you like you're flying.
By that I mean, if you look down you're going to start falling as if there would be no ladder and if you look forward or up, you slow down.
I discovered this bug by trying to skip the boring ladder ride up to my nether roof by using a rocket and my elytra and I noticed (after I screwed up the landing) that I'm falling as if there was no ladder there with an incredibly fast ladder climbing sound effect,
I found this on 1.15.2 but I tested it on the latest 1.16 snapshot as well and it does the same thing.