I think each mission pool is separate and individual.
Theory: Looks like there's 7 different pools:
"Base pool": which pops the level 90 mission (you know which mission I'm talking about) by default, except when you get one of the rare missions for things like Halaa tokens, reroll books, dance studio, hearthstone, etc. No pre-req
94 pool: pops about 6-7 level 90-94 random missions at a relatively fast rate (12h?), only if you have a sub 95 follower.
99 pool: pops about 6-7 level 95-99 random missions at medium rate (18h?); was hotfixed to stay up if you have any follower above 95 (was only if 95-99 present)
100 pool: pops about 5-6 level 100 ilvl 600 to 630 missions at base rate (24h?); get only if you have level 100 followers
615 pool: pops 2 ilvl 615 missions at base rate (24h?); get only if you have ilvl 615 followers
630 pool: pops 1 ilvl 630 mission at base rate (24h)
raid pool: pops ilvl 645 Highmaul raids at very slow rate (random?); get only if you have 2 level 645 followers (I know it's not 3, I got my first raid mission just after upgrading the second follower to 645. Upgraded, went back to mission list, raid was staring at me)
I think they implemented it like that to guarantee that you have a variety of missions available, and that RNG can't fuck you (as in "I have only one 1 level 100 follower and 16 level 100 missions and nothing else"). You could do it differently while still getting a variety of missions, but that's not what they implemented.
(my design would be: 1 mission slot per active follower, mission in that slot is a mission that's the lvl/ilvl of that follower, mission repops a certain time after being finished based on mission level, or after 72h if mission isn't started. Plus the one base slot for story missions + random rares)