velocities on Slice over polyline

Slice over polyline only returns two non-zero velocity components (Ux and Uy) when applied to my three-dimensional flow field computed with OpenFOAM. Ux is essentially normal to the slice, while Uy is one of the in-plane components. The values of the second in-plane component, Uz, are in the range [-1e-39:1e-39], but should be similar to Uy and of order 1.0. This happens with both versions 5.5.2 and 5.11.1. The values of scalar variables all seem reasonable.

The other slice filters return non-zero velocities for all components as expected.

Is this a known bug, or are there some assumptions about the interpolated field?