Well, I have now done some additional splitting of wmv/wvc1 files and this problem is obviously not common.
Actually, I could only find 2 files that had this problem.
I tried with the latest v2.3 splitter. Exactly the same result as with the v2.4 splitter.
v2.3 has an installation bug, SMM_ASF_Trimmer.ax is not installed, and I had to take it from v2.4.
I tried with different media players. Only the MPC-HC will not play the trimmed file properly.
I use the second-latest MPC-HC, since they screwed up the .mkv header flag reading in the latest version.
I don't know if this is a Solveigmm, or a MPC-HC issue, but it is obviously not a major problem,
so I have solved the problem by deleting the 2 files