That’s absolutely normal, especially as people tend to compare their general performance with their peak performance („I managed to play that without errors once, why am I not managing it every time?“).

You can do a number of things to improve your mental consistency within the limits of your current abilities by forming healthy habits around it, if it is very important. E.g. a balanced lifestyle of sorts and a good warmup routine when playing your instrument, but even then you will have off days.

If you want to nail that section consistently, you will have to increase your skill ceiling quite a bit, way beyond „nailing it once“. Usually you don’t notice that progress as much with instruments, because ideally the things you are working on are always challenging you at your current level, but try going back to something you played one or two years ago. You may have to refresh the piece until it sounds good again, but then you will probably be way more consistent about your performance than the first time around.

Rocket league is a completely different skill set, but the concepts apply in the same way. Structuring your life around being in ideal mental and physical condition to perform in a video game is probably a lot less useful for your life, but I’d bet every professional player does it.