From what i've seen monsters definitely react to their surroundings in interesting ways that affect their elemental weaknesses. You'll notice changes when the weather shifts or terrain conditions change like when sandstorms kick up or rain starts falling, which can make them more resistant to certain elements.

The key is paying attention to what's happening around you. When you see a monster getting coated in sand or taking cover in water that's usually your cue that their resistances are shifting. Focus mode really helps here since you can spot weak points better especially after big attacks like when rey dau fires its lightning beam and exposes its horn. Sometimes bringing multiple element weapons is smart, particularly when fighting something like chatacabra where thunder damage works great on those back legs when you get the chance.

Working with your team makes a huge difference too and knowing when to strike is crucial. Perfect blocks and power clashes create nice openings, and if someone's running a hunting horn they can help counter nasty effects like waterblight that might slow you down. The monsters aren't constantly changing elements or anything wild like that but they do adapt to their environment which keeps you on your toes. Just keep an eye on those environmental signals, coordinate with your group and you'll start seeing better results especially when targeting those exposed weak points after big attacks.