How do I do that? I select the first measure, and then shift-select a hundred measures later, and I kind of assume that everything in between gets selected, but apparently it does not. The measures with visible triplet markings (including the first measure) get copied correctly. I’m sure not going to try to dig into all several hundred in-between measures (a couple of hundred measures times five staves, of which probably more thab half have hidden triplets) and manually do something to each measure. It takes far less time to just fix the hidden numbers. 
Seems like Dorico badly needs the equivalent of Finale’s measure tool, where you’re guaranteed that an entire measure gets selected. In all my decades of composing and arranging, the number of times I’ve wanted to select part of a measure and do something with it can be counted on one hand. When I copy a measure, I expect the entire measure to be copied exactly, as-is, without anything changing. And when that assumption is violated, it’s a very bad user experience.