Merging creates an empty track

Hi there,
had an issue here:
Since I have used all 4 tracks and needed one more for an additional instrument I copied all tracks by shift-lifting them and dropping them to a different spot on the tape. (Also dropped them back to place before) Then I deleted 2 tracks. Shift-lifted the remaining 2 and shift-dropped them again on spot.
The new - now single - track is there (is red!) but empty. I repeated this a couple of times with same results.
Only switching the OP-1F off and on again solved the issue.
Wanted to let you know.

PS: Is there a way to merge only 2 out of 4 tracks in place without the steps mentioned above?