OH MY GOD??? maybe thats why he seems to be less in love with victor lately
maybe heart's feelings for victor are more of a habit since, like you said, it could just be that he was soft natured to begin with. this state of being in love could be more like it's the only thing he knows since waking up? and he has to rely on these feelings because he has no memory, and it seems like he's starting to realize the truth behind their relationship but he behaves the same way he always has out of habit and uncertainty.
it could also be that the 3 kept their original personalities from before they were frankensteigned, but its hard to tell when they align so well with henry's character traits. I do think it's a bit of both and in Heart's case, i think it's like you said; he's got rose tinted glasses that exaggerates his affection for victor.
i think he is so loving towards victor cause it was his soft nature to begin with, i think victor is the main center because the heart remembered him and his kindness but i doubt the original head ever loved him. based on what we see he had victor as more of admired and someone who funded his whims. which heart may be undirectly knowing of so he overcompensates with his natural gentleness / softness towards victor.
+ he noticed victors "love" (downright obsession) and he probably thinks victor is not a freak like all of em except heart, he seems to have rose colored glasses on about victors affection