In another forum (Mods) Arthmoor stated;

"...Entirely possible. If that mod hasn't been updated for 1.6 yet it may need to be. The navmesh data format changed with 1.6.89 and I can easily see conflicts with mods that have older navmesh data. Have you tried simply loading the carriage mod into the CK and saving it back out? That MIGHT be enough, but if that doesn't help, the author would need to refinalize all of their edited navmeshes. ... "

That got me thinking. I am not 100% sure I finalized all my cells post 1.6
I know I did finalize them, just not all again after 1.6

What sort of issues might this cause? Would it only be conflicts with other mods in that area?
Only with the vanilla mesh?

CTD?

Only NPC nav pathing issues?

CTDs or at least for me when I install old house mods my game would instantly CTD.