When I copy measure 45 and paste it into measure 53, the rest of the bars in the piece look like the screen shot. I can’t figure out why this happens. It’s happened on quite a few other pieces as well.
Welcome to the forum @Chris_Cash – this is because rests have been removed after the last note in its original location, which suppresses rests after that point. Without rests in the bar, Dorico has nothing to use for horizontal spacing.
You can get the rests back by selecting the last note in bar 53 and deactivating “Ends voice” in the Properties panel at the bottom of the window (if you can’t see the Properties panel, press Ctrl/Cmd-8 and click the Properties button, the one that looks like a list.)
Slash regions should automatically hide bar rests, so you don’t need to remove them before adding the slash regions. Indeed, by default they also hide notes, although you can choose to show notes alongside selected slash regions.
Fantastic! Thanks Lillie, this was driving me nuts!