That's a tough one. It's likely caused by a defect called shake, where the wood splits almost parallel to the surface. I'll guess that your floor was either water popped or finished with a water based finish. The water seeps into the defect, swelling and distorting the wood. The reason it's so tough is that it usually can't be seen until the finish goes on, at which time it's hard to fix.

We usually cut them out carefully with a sharp razor knife and use hot wax/shellac/burn-in sticks to repair. It may not be perfect after, but the alternatives are to replace the board and try to blend it, or replace the board and refinish the whole floor. I always start with the smallest, least damaging method to fix an issue.

It's rare for a floor guy to have this equipment or skill, but a furniture repair person usually does.

If this is on a new floor, your floor guy should pay for it. If it was an existing floor being refinished, I'd consider it an existing flaw in the wood. I'm fortunate to have someone on payroll that can do this, so I pay for similar repairs, but I can't say that others would do the same.

https://www.shellac.net/burninsupplies.html

Timothy thank you for your thorough and clearly knowledgeable response. It’s existing flooring, so not newly installed. And yes it was water popped and he used a water based finish. He still has to come back and put the 3rd and final top coat on. So I’ll see what he says then. Thanks for your suggestions on how to fix it. I’ll share the info with him.

The downside to water popping and water base finishes. You can use a razor blade and cut along the high spots, along the grain to loosen it up. Lift it up just enough to clean out under it. Add super glue recommended for wood or regular wood glue, whichever you are most comfortable with. Press in place with something heavy. You may have to add color matching filler or a wax stick like Timothy suggested. Aqua Coat grain filler will help also, since it dries clear.

Good luck. I'm glad to hear it's getting a third coat. I thought it looked a bit grainy. Hopefully it can be repaired and the final coat will help hide any repair work.