But, the thing is, you did set the slot to be one specific job only. It's not your fault; you had no say in the matter. When putting together the party finder, you were forced to accept that the already filled slots would be locked to the jobs that they started with.
There is, absolutely, 100%, a problem with the party finder, but the problem is not at the moment someone leaves the party, and the recruitment criteria should definitely not adjust itself when someone leaves, because some slots sometimes need to be filled by the specific job that left.
The problem is at the beginning, when the recruitment listing is created. The way the Recruitment Criteria interface should work is it should treat the filled slots like the unfilled slots, starting them as All / THDD / TTHHDDDD and allowing you to carve out unwanted classes, jobs, or roles from that starting point. Disallow carving out jobs to the point where the recruitment slots can no longer be mapped to the current party members, either by forcing the job that is already in each filled slot to be part of the allowed list for that slot, or by disabling the Recruit Members button if you carve out too many jobs to the point that recruitment slots can no longer be mapped to party members.
tl;dr: Yes, fix this; the problem is not when people leave though.