I would call this a strange bug.
I have no idea why it would occur. All I can do is provide a simplified example and some observations in case you intend to bring this bug to PTCs attention (good luck).
The bug shows up if the matrix has more than 1 column and is seen in all columns which include at least one text entry.
A crude workaround could be to always call the routine with MatrixCol.min(stack(...)) instead of just MatrixCol.min(...).
Indeed a very strange bug.
BTW, don't put M <-- stack(M) as first line in your program as if you do it will fail in any case when text is involved.
1. What service release of Mathcad are you using?
2. Please send your issue for PTC's technical support: https://support.ptc.com/apps/case_logger_viewer/auth/ssl/log