Counterargument: “a hero” and “he or she” are singular. The wording doesn’t suggest this would apply to multiple heroes any more than it applies to the active hero.
My gut feeling for the intent of this card is that it forces a monster to be placed whether the hero explored or not.
A hero is an indefinite. Hence the use of the indefinite article "A."
If it were only the current hero it would say active or the.
Regardless of intent, the wording is clear. the key is at the end of Each exploration phase.
"A Hero" can refer to multiple instances, so long as they meet the condition. I.e. They do not have a monster card and it is the end of an exploration phase.