We had a new oak staircase and bannister installed a few years ago. The carpenter used Varathane Premium wood stain and then applied 2 coats of Bona Mega Wood Floor Finish, with wait time in between the coats of Bona. There was an odor a week or two only. We are very pleased with the finish of our stairs. I have not heard of an odor lasting for years in any finished flooring. Maybe you should consult the company that installed your flooring.

The fact that the unpleasant odor is correlated with higher humidity and warmer temperatures suggests you may have mold/mildew issues. Is the engineered hardwood installed on a concrete slab? If so, what measures were taken for waterproofing? If it's not on a slab, is it over a crawlspace or basement foundation?

Is there any chance that the flooring came from a source that had smoke damage? Reaching here.

we had smoke damaged furniture and other items from a fire. the items smelled really bad when humidity was high (ie; rain, etc etc). We finally got rid of those items. the odor was horrible.

@rainbowbee How is your Havwood flooring holding up? Would you recommend them? Thank you!

It's not the flooring. I suspect that the subfloor under it is the problem. Doesn't matter so much if it's cement, crawlspace or full basement. Moisture is ccoming up from the ground and entering your home. A good vapor barrier is needed. Really doesn't matter now unless you can get to it. An unfinished floor in a crawlspace or basement would be accessible and ca nbe remedied.

Or not enough ventilation in your home. Excess moisture will be absorbed in the wood and after 3 years it could be high. A fiddy dollar moisture meter may help you diagnose the issue. Very easy to use, hold it to the flooring for a bit.

Just my opinion based on my experiences. I would buy a moisture meter and go investigate everything you can get to. Good luck n have fun