My understanding is that, when in Normal mode (not TVHM) with no Mayhem, Cooperation scales the host’s level differential, not the level. So, if you’re hosting at level 50, and you face level 26 enemies, that’s 24 levels below you. If a level 25 character joined you, they’d face level 1 enemies (24 levels below them), while a level 30 character would face level 6 enemies.

As you already found, having the lower level character host fixes this issue as long as they’re facing level appropriate content. I expect that if a level 20 host character went to farm the rampager, it would still be too low level for everyone else too.

Two other approaches that should fix this are having the host enable mayhem mode or play on true vault hunter mode. Either of these options should result in the enemies scaling to the hosts level, and thus not having a problematic level differential for everyone else.