Same here, even in 2019 on the 4.22 version. The sphere of painting foliage has disappeared. I can’t paint foliage on the landscape.
I did not tweak many things in the landscape and it was not imported. Strange. The suggestions above did not worked.
No choice than to delete landscape and made a new one in the same map. The foliage tool reappear.
I would advise to start putting some foliage before painting all your landscape layers just to be careful.