When I reach above 18.000 feet, I can't control my speed anymore on the AP. Does anybody else have that problem too?

[FlyingDwarf]

When I reach above 18.000 feet, I can't control my speed anymore on my Automatic Pilot, Does anybody else haves that problem too? And knows what to do about it?

Thank you very much,

Greetings,

FlyingDwarf (The Netherlands)