Assuming you know the point of intersection (the corner) between PB and PL (let's call it PI), you can calculate a vector V = (P - PI). If (length(V) < sphereRadius) your point is inside, otherwise, check the dot product of normalized(V) with PB and PL plane normals, if and only if both values are negative, point is outside, if the comparison fails, it's inside.
Take this with a grain of salt since I've never tested or even considered this case when doing frustum culling, but this would be my first guess to solve this issue.
Hope it helped.

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