Nexus Mod Manager is broken. Use Mod Organizer 2, if you want the best Mod Manager for FNV. Also FNV has an engine bug that exists since Oblivion where it starts breaking before it reaches the maximun 255 plugins.
But you can use "FNV Mod Limit Fix" to fix it: https://www.nexusmods.com/newvegas/mods/68714 It also has the side effect of increasing game performance a bit, so it's pretty much an essential mod for FNV, no matter what other mods you want.