Strider is not standing on the lava, they just float on the lava (not touching the lava). Because of this, the strider can't walk that fast on lava, their speed is just the speed when they walk on land. and if they somehow go up a block when this bug happen, they just float in the air and they don't go back down. The first time I encounter the bug is when I go up on a block when to transport them to the strider farm, then they just float at the height of the block.

I've uploaded some pictures that shows them floating, there's a picture that shows how their feet are supposed to be in the lava, and when they are just floating on the lava.

I hope this helps you guys for the final release of 1.16.