Repeatedly trying to mount a saddled strider while sneaking will appear as though the player hasn't mounted the strider although they actually have.
Seems that breaking blocks while in this state generates block lag, pressing the dismount key once again will cause the player to suddenly appear next to the strider as though they just dismounted it.
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You can use this bug to instantly teleport back to the location after a journey by pressing shift
This is the same issue as MC-173404, resolving as a duplicate.
Try to mount the Strider with shift button (sneaking) pressed.
You willl be dismounted.
Keep shift button pressed, and try to mount the Strider again.