Finally, there is a solution that works and does not re-encode MTS (M2TS) files
Open the first file with the ADD button, the rest of the files with the APPEND button
And then save to m2ts and works perfectly.
Be careful about A/V sync issue. If you start join a lot of clips, the later ones' audio can get terribly out of sync with tsMuxeR. Pixela, on the other hand, has no such issue.