The bug

If a stack of entities' height exceeds the maximum length of a lead (such as 5 or more adult Striders), riding the top one while leashed to the bottom will pull the bottom entity into the air. This, in turn, pushes the entire stack, including the player on top, further up. This cycle repeats indefinitely, causing the stack to fly higher and higher into the air.

How to reproduce

Summon in a stack of 5 or more adult Striders (such as with the command below) with the uppermost Strider saddled. Attach a lead to the bottom Strider in the stack, and ride the top one.

/summon minecraft:strider ~ ~ ~ {Passengers:[{id:"minecraft:strider",Passengers:[{id:"minecraft:strider",Passengers:[{id:"minecraft:strider",Passengers:[{id:"minecraft:strider",Saddle:1b}]}]}]}]}

Update

This bug was originally found using the Striders added in version 20w13a, because of their natural stacking and rideability. But, as pointed out by Marcono1234, this bug is present for any stack of entities. After some further testing, I've found that this bug was present back in the first snapshot the lead was added, 13w16a.