This one is definitely a bug and is something that was added to live some time ago.
I remember very well back in Vanilla after buffing someone, throwing a heal over time or larger heal on a player after buffing them. It didn’t matter if it was a player that was at full health I buffed in the city or what, I always saw their health bar go down and threw a heal on them to compensate for it.
I really hope this doesn’t stay in and they revert it back to how it was. Sounds like a small thing, but, I would much prefer it revert back to how it was.