I get this issue a lot when merging M-enabled lines in ArcGIS Desktop 10.8. I have ensured that the lines are both oriented in a consecutive direction, that the common endpoints are snapped to each other so they have the same coordinates, the common endpoints have consistent Z values and consistent M values. Still, I sometimes get a multi-part result. I have found a couple other things that have worked for me:
- Before merging, ensure you have saved any edits.
- Before making needed edits and merging, if your data frame is in a different coordinate system from the line data you're using, change the CS of the data frame. I'm not entirely sure why this might solve the problem, but it did for me once when nothing else worked.