I had the same issue occur approx. 2 weeks ago when backing into my garage. Had to turn ignition off and restart a few times to get the guide lines to reappear. This was after the backup camera screen failed to appear when in reverse. To reiterate, first no camera, then no guide lines.
Haven't driven it since then so not sure if this will reoccur. I guess I'll find out this weekend. Mine is not a daily or even weekly/weekend driver. Purely for occasional weekend use. Six months of ownership and only 400 miles on the odometer. I'm hoping it was a one time glitch!
They’ll be back. Never know when things will work or not or reappear. I don’t know why. And y’all know it’s not just the lines! Mine did that yesterday, and I freaked out. I have to have my lines. Please don’t ever ask me to back a trailer. Total PTSD!