Eldritch altar mods only apply to a single boss on multi-boss maps

When their are multiple map bosses in a map, only one of them gets the bonuses from the Eldritch altars. I've noticed this on natural multiple boss maps (ex. Promenade) as well as on duplicated bosses from the "Enriched Entities" atlas passive.

In this clip, map bosses should drop 3 scarabs each, but only one set dropped.
https://youtu.be/bRPgCNINDvY

this is intended and not a bug.