Did I just step on a bug?

This is a piano piece. Being lazy, I am using the 'N" key to move notes from the bass to treble clef. I would like to use the 8va sign with treble clef notes, but when I do, the notes moved from the bass clef are also transposed an octave lower.

I tried using different voices, but with the same result.

I don’t think you stepped on a bug, no. If you use N the notes are still attached to the lower staff, so any 8va line you add will be applying to the lower staff even if the notes themselves are appearing on the top staff. Use Alt-N to actually move the notes to the top staff, rather than N to cross-staff them.

Incidentally if you do need to apply an octave line to some cross-staffed notes, you can place a voice-specific 8va line from the caret (with the caret in the correct voice before launching the popover, and confirming the popover with Alt-Enter). Without so much as a screenshot I can’t be sure whether that’s necessary here.