That’s going to depend on how much temperature drop there really is. At that stage the eggs are generating a lot of heat on their own. They generate so much heat that a problem with the big commercial incubators where they may hatch 60,000 at a time is to get the extra heat away, not provide heat, in the later stages.
I don’t know how much of a drop is safe. If you have a commercial incubator and the turner that was made for that incubator, I’d guess it is not a problem at all. You can read the literature that came with the incubator and see what they say about how you go into lockdown. Or you can contact them and ask directly.
I think it is a reasonable question but I also think you may be over-thinking it a bit. I tend to do that too. This question practically never comes up and a whole lot of people use turners with still air incubators. If it were a problem it would be discussed in depth on here. But to make yourself feel better, you can contact the manufacturer and see what they say.
Good luck with the hatch.