I found four bugs in Dorico 5.1.40.2138:
(1) Make a dotted 8th note rest (i.e., duration 3/16), select it, go to engrave mode, then in the lower zone (command-8) turn on the “Common” → “Offset” slider, and change the Y-field to move the rest vertically. You will see that the body of the rest moves, but its dot stays in place (it shouldn’t).
(2) In a fresh piano bar, create two consecutive quarter notes in the left hand. Add a grace note that precedes the second of the quarter notes. Finally, try to add an “8ba” instruction to the first quarter note ONLY, by selecting the first quarter note, then in Write mode (Command-2) in the Right Zone (Command-9) clicking on the clef symbol and then under “Octave Lines” clicking on the bottom left (8ba) symbol. Unfortunately, this (8ba) instruction appears to also apply to the grace notes following the first quarter note, and I could not manage to tell the (8ba) line stop earlier, so that it only applies to the first quarter note.
(Note: My example screenshot has different note durations, but that is irrelevant. Also, I know I can cheat it by shortening the (8ba) line in engrave mode, but then during playback the (8ba) still applies to the grace notes anyways, which is not desired.)
(3) Put a horizontal line underneath a note, as seen in the uploaded image; by selecting a note and then in Write mode in the Right Zone clicking on “Lines” and then on the horizontal line that has both its ends pointing downwards. Select the newly added horizontal line, and then in the Lower Zone set “Horizontal Lines” → “Placement” to “Below”. As a result, the ends of the horizontal line now point UP, which is correct. However, if you now go to Engrave Mode and try to move this horizontal line by dragging it, you will see that DURING the dragging operation, the ends of the line briefly point downwards (!) and only after releasing the mouse button they point upwards again. So while this is not really an issue preventing me from creating the sheet music I want, it is a graphical bug during the engraving process.
(4) Why is it that when in Engrave mode you are zooming in all the way to edit details, such as the exact shape of a slur, Dorico increases the size of the red rectangle handles as well?? They should stay the same size on the screen, so that when zoomed in they become smaller relative to the notes. The way it is right now, if a slur is really small and I cannot easily select the specific red square handle because they are all overlapping, then zooming in doesn’t mitigate this problem. This seems not really a bug, but like a really poor design decision.
I hope this helps in your development process. Your software is awesome.