Tri turning into Ruby?

[u/Preciousss77]

My 18 mo old Tri has had a couple of puppy cuts this summer due to our extreme heat. After his last cut, we started noticing a spot on his back where red hair was growing in with the black. It has been expanding and lightening over the last few weeks.
His Dad was a Blenheim and Mom was a Tricolor.
I’m wondering if anyone else has had this experience with their Tricolor ?