Answer:

That has to do with the way Movie Render Queue works in UE5 now, just in general not really anything project specific.

Basically for whatever reason, Epic set up MRQ by default to try and load the ENTIRE grass across the landscape at once when rendering out a movie in Movie Render Queue. Now, I mean, that might work by default for a smaller map – maybe like 2k in size or smaller, but it’ll crash a larger map for sure, since a computer can’t load probably millions (billions?) of blades of grass across an entire landscape at once without crashing or culling in the distance.

No problem though, there’s a setting for it.

So the way to get around this is, when you’re in MRQ, click on “unsaved config” then go up to “+ settings” and add “Game Overrides”. In there, untick “Flush Grass Streaming” and your MRQ will stop crashing and you can fully use any grass. Here’s an image for reference