Growing in my yard I didn't plant it.

If it set itself from seed it won't come true to a cultivar. Do you have any Hibiscus syriacus?

How about "red heart"?

At my house, the seedlings look the same as the parent tree...

Although... I do have the lavender "Minerva" (also), and there are slight colour variations

I have Hibiscus Red Heart, the petals are much fuller and overlapping with a slightly bigger red eye.

It's close to Lil' Kim which is a 'proven winners' cultivar but there has to be more similar cultivars, and if grown from seed it couldn't be given the cultivar name.

https://www.provenwinners.com/...

JRsbugs said:If it set itself from seed it won't come true to a cultivar. Do you have any Hibiscus syriacus?

My neighbor used to have rows of them in various colors but they are gone, so mabye this is from seeds of those?

That sounds highly likely! A free gift like that can't be bad.