20 min before the pickup time you receive a request from Lyft to go ONLINE for a scheduled ride. After going ONLINE, you may receive a request from Lyft to accept a different ride, which is not the one you're waiting to accept. Most of the time that happens either in case your scheduled ride is already canceled by the passenger, or if you're more than 20 miles away from its pickup location.
So, when you get a request from Lyft to accept a ride, please check if that request has the text "Your scheduled pickup" above the "Accept" button, as follows:
If the request does have the above text, then it is the ride you're waiting for, so you should accept it.
But, if the request does not show that text, then it is a request for an extraneous ride, and it is up to you to decide if you want to take it instead of the scheduled pickup you're waiting for.
The worst part about not accepting this extraneous ride is that ignoring it may affect your "acceptance rate" on Lyft.
As per our knowledge, this glitch happens because the Lyft system does not exclude you from the list of available drivers when you go ONLINE for a scheduled pickup.
Unfortunately, we are unable to provide any workaround for this case, so if your "acceptance rate" was affected, please contact Lyft Support.