"Keep everyone together" explanation?

For the past month or two a group I ride with have been using the “Keep everyone together” feature on meetups and it’s been useful when riding with riders of differing abilities, especially when we’re on a route with longer climbs.

The past couple of days we’ve noticed behaviour that seems different from how it used to. It seems that there is a specific rider/order to the bunch which is very hard to change. For example, on a longer climb there will be a rider at the front that is very hard to catch, let alone pass. He might be doing 3w/kg with a few metres gap to me. If I do 5-6w/kg to close the gap, as soon as I get close he suddenly speeds up (or I am slowed down) and the gap is there again. The same thing for the rider behind me.

We are fairly certain it didn’t used to be like this until a few days ago. We could ride more like a bunch, if you were doing more w/kg you’d get ahead a certain extent (only a couple of seconds) but the bunch would generally be ordered in a way that made sense according to our power/weight. This no longer seems to be the case.

Can anyone explain the behaviour, how it works, if there have been changes recently just so we have an explanation for what is going on?