Frontier, along with the other LCCs, like Spirit, have this happen a lot more often.To be fair alot of Airlines have this issue. The problem is that once you're International all "employees" except the flight crew are contractors. Even in the US all mechanics are contractors except for Denver and Phoenix. Id call Frontiers customer service number. I worked there for seven months until covid hit.
Your generalization about international employees applies to Frontier.
Not the majors.
Frontier merging with Spirit will, no doubt, make this worse in the short term.