When a sentence is split between pages, it’s impossible to select multiple words at the end of one page and a few at the start of the next page. The selection from the previous page is cleared when flipping.
Maybe display the previous page’s last line as the new page’s first line? That wastes space, though, maybe there are better options.
I would hope there would be a better solution because one of the things it tends to do correctly is show you the next word you’re supposed to read when you turn the page. It would be a shame to lose that.
When this happens to me I either show/hide some definitions or change the font size to force together the text I need…
I also sometimes flip the page back and can then select the phrase when the highlights are no longer taking up space.
Another workaround could be to select any part of the phrase and then edit it and copy the missing part manually.
It’s both a little tedious and it would be great to have a proper solution (but I’m not even sure what I would imagine it to be), but at least your don’t have to miss out on anything if you apply one of the workarounds.