Question - I recently noticed what sounds like a hollow spot under the hardwood floor when entering my master bedroo (I'm honestly not sure if it's hollow since the home is literally less than two years old) Literally the board that separates the bedroom from the hallway. If I walk or push hard on the board I can slightly move down one of them. Very slightly, but noticeable.

My home is currently low 30's humidity and I'm wondering if come spring/summer if this joint will expand becoming tighter and enough to not make it move.

I was considering injecting either glue or another solution under the board but again if this is a humidity issue I wont do that.

Thoughts?

The wood needs to contact the surface below it to be firm enough not to move. Injecting more adhesive near the end will keep it in place. Now or later, it will need to be done.

Is it ok to do this on real hardwood (not engineered) that is over a subfloor?

You will need to drill a tiny hole to inject more adhesive where it moves

Thank you .. but is glue adhesive a problem for the shifting of the floors? Or should I use a more flexible adhesive ?

Gluing on or more boards will not reduce the integrity of the floor. I recommend the 3M epoxy kit that includes a two prt epoxy in a tube that can be injected through a mixing tube that has a nozzle that fits into a hole drilled by a supplied drill bit and also includes the gun. You can also use general purpose epoxy, a two part mix, with a syringe. The syringe must have a tapered tip.