That isn't enough information.
That's not a bad opcode so the debugger is not snapping on that line. And even if it were, 1 single instruction wouldn't be enough to know what is going on.
I'm also not very familiar with the game so I'm not sure what's going wrong in the video. I assume the player shouldn't be jumping to the top of the screen?
Anyway... aside from graphical glitches... actual gameplay-altering problems you can have would be one of the following:Possible
: PRG-RAM is not enabled and the game expects it to be (see $5102, $5103) ... though I'm not sure if Guardian Legend uses PRG-RAM? **shrug**Possible
: You didn't disable mapper IRQs (see $5204). IIRC Guardian Legend uses DMC IRQs for timing tricks -- particularly during the flying sections.Possible but not very likely
: You're in the wrong PRG mode (see $5100)Much more likely to cause a flat out crash than a weird problem
: You're doing PRG swapping incorrectly (see $5113-5117)
Borked CHR swapping could also cause gameplay problems (if the game uses Sprite-0 hit, which IIRC Guardian Legend does) -- but that would more likely result in a crash than anything else, and would be accompanied with lots of obvious graphical glitches.