One of my Barn Find that i discovered just disappeared from the map, anyone has the same problem?
I had the same experience, just go to your garage and you will see a option called barn find thats where the vehicles are located after they are restored.
What should I do if I have only done 11 of them and the big purple circles have disappeared?
“Instead, you must wait until the game tells you about them - by simply playing other activities in Forza Horizon 5 - where it will then give you a vague pointer of their location on the map.”
Just keep playing
With the radio on, right ? It’s been a while 
Hmm.
Samsies. Makes sense the radio would be on the alert but honestly can’t recall!