Copy tracks multiple times in song mode issue

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### System Information

Microsoft Windows [Version 10.0.19045.4894]

### LMMS Version(s)

Version 1.3.0-alpha.1.693+g7d35d4225 (win32/x86_64, Qt 5.15.6, GCC 9.3-posix 20200320).

### Most Recent Working Version

_No response_

### Bug Summary

Hi, when selecting multiple tracks in song mode, either bij CTRL-A or the selection sqare and set the cursor at the end of the selection, then it is not possible to copy paste this multiple times after each other to create a long song. At a certain point, the selection is copied at the wrong position.
The frustrating thing is that you cannot undo it with CTR-Z. This is really an irritating issue.

### Expected Behaviour

The copy should succeed multiple times

### Steps To Reproduce

So here is the selection

Then I hold CTRL and left click mouse to copy pattern to position 3, this works fine. So now it looks like this:


You see that the copy is automaticly selected. Now you repeat ctrl mouse key and move the tracks to position 15. Somethimes this will go fine one times, sometimes multiple times but at a certain point the location where you past the selection is moved to the left and is paste at the wrong point. You can reconize this on the following picture if you look at track 3. A complete empty bar is missing at postion 10. (marked red oval circle)

### Logs

Click to expand

### Screenshots / Minimum Reproducible Project

_No response_

### Please search the issue tracker for existing bug reports before submitting your own.

- [X] I have searched all existing issues and confirmed that this is not a duplicate.

Start in Song mode and reproduce the issue by selecting multiple tracks, copying them repeatedly with Ctrl-click, and checking the placement around position 15. Investigate why repeated copies shift left and why Ctrl-Z cannot undo the result. Done means repeated copies stay at their intended positions and the operation remains undoable.

Written by the indexing model from the issue text.