UPDATE:
I have tested this on 20w15a again, and somehow the issue was fixed, at least based on my tests (same setup, same method of testing, different results). I'm not sure if the fix was a result of some other bug being fixed or if it was an intentional fix for the original behavior, but Striders now get their regular lava speed back even after they walk over normal blocks.
If any moderator, helper, or Mojang employee sees this, would this be grounds to mark this bug as "resolved", or is further testing required, just to be sure? This is my first bug report so I'm not sure how to contact anyone important.
The error itself is caused because the command adds "@p[x=-1103,y=52,z=450,r=2]" to the player-list of the team and interprets it as a player.
but since a player's name is only allowed to have at most 16 characters, it outputs an error.
Hello there! I have done some testing again in the latest snapshot (20w21a), and the issue seems to be fixed. I first found a naturally-spawned Strider and tested its movement speed on land and on the lava without the speed boost from the fungi-on-a-stick item, and then again with the speed boost. I also tested a Strider spawned with an egg just to be sure, and finally, I did both tests in both creative and survival.
All four tests yielded the same result, as far as I could tell: the movement speed bug appears to be fixed now, and the visual glitch that came with it (the Strider floating a little higher in the lava than it normally should after leaving the land). They not only retained their normal speed once they got back into the lava, but they also retained the speed boost as well if they had eaten from the fungi before walking onto land.
Thank you so much for fixing this bug. 🙂