The ‘Join ongoing Matches’ does not allow you to replace a leaver, it also only lets you join into Matches which are only ongoing for like ~one Minute (not sure the exact amount).
For example if a Match starts right at the beginning with a 16vs16 and four people leave in the first Minute no one is able to join into and replace them. Only when a Match started when its not full or someone who didn’t even spawn in the first place which im pretty sure also only works when that person did immediately return to Hangar before spawning once in the first 30sec-1minute else not.
Of course some time limit needs to be put, else it would be kinda awkward to join into a lost Match because someone who remained alive for long now leaved after eight Minutes or so - which is something other Games for whatever Reason can’t do right either so you quite often end up joining into Matches which are basically done for lol which just sucks as well
I’d say let people join and replace leavers in Matches when the Match hasn’t been going on for like ~3-4 Minute and or the Tickets have reached some treshold.