An update about the original question, after more torough tests:
- the problem I got initially was caused by my own code, sorry about that.
- however, If this can help anybody, I discovered that piecewise linear TFORM behaves strangely if control points describe a non-convex polygon, creating triangles with zero area in the output space. These triangles are transformed to a segment that seem to "attract" even points outside the cp polygon. I don't have the skills to test it properly, but this may suggest that piecewise linear transform might be unreliable in some cases, e.g. for processing images with barrel distorsion.