Well after further testing it does break after you catch all the possible monsters for that zone.
Viewing post in Yal's Monster Collector Engine comments
One way to fix that is to add some code setting the step range really high if the list of monsters is empty:
if(array_length(global.encounter_valid_monsters) < 1){ global.encounter_step_range = [9999, 9999] global.steps_to_next_encounter = 9999 }
For additional stability, go to player_step_fill and add an if statement around line 10 to not count up encounter steps if the limit is 9999:
if(global.steps_to_next_encounter < 9999){ encounter_steps++ }