The effect of transfusion has not been tested and is as implemented not working correctly. The issue comes from the healing being linked to the rez port, which is affected by target cap. That means that if you have 5 people that are down in the other subgroup and all people in your subgroup have 99% hp transfusion will not port a single person.
ALL other rez abilites and rez traits (like search and rescue, signets or nature spirit) on the other hand will always port the downed target regardless of subgroup since they only target downed people specifically.
Before the change that wasnt the case because the port was a permanent effect that affected downed allies, split off from the healing ticks (which sometimes mean you ported someone without healing them with the transfusion affect due to the healing going to target cap). Every person had an internal cd but the port itself did not port on ticks, but as a constant affect.
With how it works now its incredibly frustrating to use since even if you see 4 downed people and use the skill, you could just port no one to you at all negating the reason to take the trait in the first place.
Fixing that would be as easy as reverting the change and keeping the rez tied to a different internal effect that, if you want to nerf the streng of skill, could only last for one second instead of the old 6, keeping the effect only relevant directly after pressing the skill without the extreme frustration of the current implimentation which makes it extremely unfun to play (wasting the rez on healing fulllife people instead of porting the downed people is a horrible design decision)