World created in 1.14.4 when that was the most recent official release, then updated to 1.15.2, then 1.16, then 1.16.1. Equipping a saddle to a strider and then riding it with the warped fungus on a stick in hand does not result in me getting the advancement. The area I tried to do this in was generated in 1.14.4, maybe that prevents the advancement from triggering for some reason. That was my initial thought, but, after loading a backup, retrying and failing, exiting out while on the strider in lava, then logging back in (singleplayer as always, not on an online server) it triggered. Before it triggered, though, I immediately caught fire after logging on while on the strider in lava. After guiding the strider back onto some netherack with the warp fungus on a stick and eating some golden carrots, the advancement triggered. The most obvious guess as to why that happened is some kind of lava glitch/lag related to loading the game while on a strider in lava. Other potentially relevant details- ()The strider was in the middle of what seems like a deep lava lake when I equipped the saddle to it and tried and failed to get the advancement the first time. Perhaps this had to do with the strider not being connected to any solid blocks. Maybe there also is a hidden trigger I don't know about that only triggers the advancement after transitions from block to lava and from lava to block have happened. Yes, I had been using, or switched to and back from, the warped fungus on a stick during all of this. With the way the advancement finally showed up, it looks like some sort of lag could have been the issue, however, the time I spent trying was not insignificant, so it would have had to have been some intense lag.