Nissan Patrol 4x4 (Africa)

Good afternoon and seasons greetings to all.
I opened my fuel filler cap today and was greeted by a serious "whoosh!" The fuel tank had some serious pressure in it, took a few seconds for the pressure to drop.

Is this normal, could it be the cause of my missing and the engine not wanting to rev?

Definitely not normal or correct, and could cause issues (although normally it's vacuum that causes problems, not positive pressure).

In the wheel arch under the filler there is a flimsy plastic cover held on with 3-4 screws. Remove that (can be done without removing the wheel, you just need a stubby screwdiver). Under that you eill find the breather. Make sure it's open , clean and working.