Bug? Animal's Hearts Can't Go Higher Than 5

[kshatriya]

Hello everyone!
I'm currently in my 3rd year and have been owning my animals since the 1st year.

However, all of my animal's (chickens, rabbits, cows, sheeps, and alpacas) heart growth seems to stop at 5 and can't go higher than that.

I first noticed the anomaly when I was invited to the contest and saw that the hearts should be 10 at max. So it kinda looks like this.
♥♥♥♥♥♡♡♡♡♡
It remains the same for three years.

Did I do something wrong?
I unfortunately didn't let my animals out for the first couple months since I thought I needed to push them to the barn, like the original game. I just realized that they would automatically go back after 8 PM.

Does anyone have the same problem, what should I do?
My pet's (horse, dog, and capybara) hearts are fine and they are in maximum 10 hearts.

Thank you!

[Mewtwomew]

Breed them. Only after a few generations of animals you will get full 10 hearts, so far I am at 7-hearts chicken.

[kshatriya]

Oh I see! Yes that makes sense. Thank you so much.