I bought 30min/30% zone boost then 60min/50% zone boost in a party of 4.
When there was about 10min left of the 50% zone boost, the spawn rate was exceeding that. It was even faster than the dynamic spawn rate for 4 players.
It looks like zone boost doesn't account for dynamic spawn rate and maybe that's intended but the bug is inconsistent spawn rate.
Death of first mob: https://www.twitch.tv/videos/2680207932?t=01h33m25s
Respawn: https://www.twitch.tv/videos/2680207932?t=01h37m40s
Default spawn rate: 660sec per https://wiki.medivia.online/mapper/?x=2594&y=-2604&z=7&zoom=3
50% zone boost: 330sec
Observed rate: 255sec
You can see before that we're waiting 2min each lap, then our exp/hr jumps 1kk and there's no waiting