Had old floors Refinished and some new hardwood installed. Where they meet is an obvious difference. Both are white oak with golden oak stain. No poly yet in these pics. Any thoughts on what the issues is?

https://imgur.com/a/CUP6MNY

Oak and other hardwoods, along with other wood species will amber with exposure to light, which deepens the color. Ambling over time will affect not just the very surface, but to a depth that light can penetrate so sanding will not always remove the ambling effect on older floors. Absorption of stain can be influenced by where the wood is from forest to forest. This color difference was probably apparent at the time of stain application, while the stain was still "wet". That would be the time to adjust the color by the floor mechanic, if adjustment could be [possible.