Having quest instances bound to the lobby they were created in worked in PSO1 and PSU because there was only ever 1 party of players involved with any given quest, and all quests were available at all times of the day. But now that PSO2 has the multi-party system, as well as the time-limited Urgent Quest system, it has exposed some critical problems with the block/lobby system.
To clear an Urgent Quest in the ideal amount of time, or reach the ideal score for the quest when it's time expires, You generally want to have the highest number of players the quest will allow; which can sometimes be an issue when everybody slams the "quest start" button at the same time when an Urgent Quest begins and you pull the short straw and land in a 5/12 multiparty.
The multi-block matching option serves as a bandage fix for the situation, but Premium Block Space (it wasn't available in closed beta, but it sounds like that'll be on it's way eventually) presents a wrench in that system, since a premium player could multi-block match into a 4/12 group in a block that has ran into premium space, and the other players still looking for a group don't have premium, so they'll never get matched into your multiparty.
I know this isn't really much of anything that anybody can really do about at this point; but I feel like pso2 has encountered many issues with the negative interactions between having quest instances bound to the lobby instances they were created in and other game systems. Most of those issues wound up being dealt with in roundabout ways (see: needing a Ranger in your 12 player group back when they first released the Magatsu Urgent Quest on the JP server leading to them having to reduce boss hp across the board and reduce the effects of one of Ranger's most important class skills, since they knew they had no way to guarantee that they could match a ranger into any given multiparty) but I just saw the closest thing I've ever had to an official channel I could vent this frustration into without having to ask somebody to translate my text so here we are.
tl;dr: having the lobby channels are nice and all, but I wish party instances weren't bound to the lobby channel they were made in. but there's probably nothing that can be done about it at this point.