I have written a line in 5/4 time and applied “medium swing 8ths”. It starts with an anacrusis. The four notes in the anacrusis are written using the exact same timing as the last four notes in each of the following three bars. However, the notes in the anacrusis play with very different timing. The first note is twice as long as written and the next three notes play together almost simultaneously. Both the notation and MIDI piano roll look correct.
I don’t hear exactly what you describe, but it does sound as if the swing is “backwards” in the pick-up bar. Dorico doesn’t handle every pick-up length in every meter correctly when it comes to swing, which is something we need to improve in future versions. For the time being, you could try having a full 5/4 bar instead of a pick-up, use a bar number change to number it as bar 0 (as it would be numbered if it were a pick-up), and remove the unwanted rests at the start with Edit > Remove Rests. You can close up any unwanted space left behind using the note spacing tool in Engrave mode.
I was thinking one might compensate by altering the notes of the pickup measure in the Key Editor, but I see that swing is applied elsewhere. I suppose in the overall scheme of things, not changing the note display in the Key Editor makes a lot of sense for the program overall.
