I had a similar situation. I've got one of those Garmin mounts that got recalled for the center piece popping out. Mine seemed more than fine, so I never bothered with it. When crossing over the rumple strip to the left side of the right lanes on the first trip back to Whistler, the bike vibrated so hard that my garmin 510 popped out of the mount and flew up and hit me hard in the face/helmet/sunglasses. I use that elastic leash string thing just in case (lost a powertap computer once) and the garmin swung out and down, banging into the headtube over and over again. Was definitely a suboptimal feature of my ride that day. I'm pretty sure that a bike computer swinging wildly from the bars is not very aero.
I kept trying to wedge it back in, but every big bump would pop it out again. Finally, some sugary stuff got on it, made it sticky, and it held for the last 30 miles or so. I might superglue the backing plate back in to see if it stays. The bolts for that mount are sweat-rusted shut, so I can't send it back without hack sawing some things apart.
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