[intakexhaust]

Difficult to know exactly whats causing the change but usually a milky haze is from moisture entrapment. Whats the environment where used? This perhaps also from being wet from something obvious (ex. glass of water spill) to other less obvious as in a higher humidity area. If the case and being you top coated with a polyurethane, that should have sufficiently sealed it.

Other thoughts. Is there some reaction happening from the stain and what if any recommended type of top coat? Guessing the stain could be some type of water base and for wood to constantly breath, rather than seal. Some are not compatible with a urethane.

To fix? Tough call without seeing. Depends how picky you are and what you wish to achieve in a final appearance. You might have to fully strip it, and start over. Hopefully this milky haze isn't too deep.

I'm not a fan of messy chemical stripping and usually rip at it with a variable speed orbital sander. Connect a vac to the sander. If you don't have one, Harbor Freight is the DIY deal. Try a 150 grit (though might need a 100) to cut thru the topcoat. Final and prep sand to 220 grit. Wipe with a tack cloth.

Try Minwax oil based and hand applied stain to the desired effect. Let it set overnight. For the polyurethane, brush on a light coat and let it set for another day. You'll probably see some pinholes or bubbles. You don't need to get too fancy but use a 3M scuff pad - green and go with the grains. Tack cloth off the residual and do another poly coat. If necessary, repeat a third time.

I'm a fan of spraying two-part urethane's. HVLP sprayers are the ticket. Atomized material thats less wasted in the air and gets this nasty stuff down to the surface. Can touch by hand in a bit over 15 min. Can also sand by orbital with a 220 grit in a half hour. Then spray a second coat and its usually done. Fast job.