Like https://tools.ietf.org/html/rfc1459#section-4.4.2 & https://tools.ietf.org/html/rfc2812#section-3.3.2 say should be used.
This is per-channel, because of https://github.com/42wim/matterbridge/issues/1393 and surely getting a new PM buffer in some clients or consistency, I don't remember the other reason.