I found out that, since the seller special animation has been introduced lately, if in my save I already have bought all the items from that seller I can't unlock that seller's animation. Is it a known problem ?
but Arthur, Rafaelo and Bandit are still missing the heart icon 
The above was written before what i'm writting now, I had the idea after writing all this to battle one of them and see if it fix the heart thing, I went to Golden Sand and got to battle Bandit, then I went back to his shop and the heart was there
I'm guessing the update need the player to "refresh" the affinity gauge by battling them once to get a bit of affinity point and understand that they're already full ? Posting this so people who come to read this later might have a clue if they got the same issue as me after the update
Edit : I totally forgot about the dicktionnary when I had the idea, but I just tried and renting a room to battle them anytime works too
Liam was showing just fine for me, if it really is a matter of refreshing the gauge by buying something or battling them, Liam's probably the easiest since his beer are available for free, it's harder for character that sell their goods only once and after that you can't buy anything from them if you already bought everything
If you bought the game here on itch.io, you don't have to buy it again.
Follow the steps in this link to download an update:
https://itch.io/t/1128078/how-to-download-the-updates
And since you're on Android, make sure to use the cloud save to avoid losing your save.
