I Think This Counts As A Bug
Played a few games as Singed with Aery and Font of Life. Aery never gave my teammates a shield when I marked them with W or when I had 
slow but they were definitely being healed
2 Comments
It probably prioritizes damage over healing
You need to actively target an ally with a shield or ability if I'm not wrong. Morde W will activate it if you target your ally, but the heal from Rakan's Q doesn't proc Aery since it heals in an AoE around you without you having to do anything.