Tangent arcs with a "convex" curve - strange behavior

not exactly:

this topic:

given: 2 circles, Radius R
searched: a 3rd circle / arc with Radius R, tangent to the first 2 circles

the linked topic:

given: 3 circles
searched: a 4th circle, tangent to all of the first 3 circles.

but it s great to have those “circle with condition… searched” topics linked.

cheers - tom