Excuse my cynicism...

Dealers will be Dealers all day long, they feel no obligation to Yamaha or to you.

I had a similar experiences with a Guitar Dealer, and a Brand New Fender Stratocaster bought from them - they had their "Fender Licenced Tech" have a look at it. I'm pretty sure, to be blunt, there was some corrupt arrangement there to make sure I got neither a refund or a warranty repair. They told me there was no fault, or at least, it worked within "acceptable" limits, and quoted me $600 to make improvements/upgrades (the guitar cost $1800).

Being from the UK, but living in Australia, I took the guitar back to the UK where I knew an excellent, reputable luthier/tech, who also held licences for Fender, Gibson etc. He also had "no dog in the fight".

He had it for a day and called me to say the truss rod was broken, he could make an invisible repair (no need for a new neck or guitar) and it would be charged to Fender MIC since I had my original receipts and registered the product warranty.

Lesson learned - do your research, and find an independent tech with an impeccable reputation for quality and honesty. Be prepared to pay a Bench Fee to let them diagnose the issue.

In your case, I would take it away for a second opinion and maybe drop Yamaha an email explaining the scenario and your reasons.

Thank you for sharing your experience Antony and thanks for the advice too. I think your recommendation is right.

I am still hoping to receive some more response from other MOXF owners. Would appreciate hearing experience with the same issue, and if dirt can be the root cause of the keybed problems or if it is just a matter of bad quality.

Hi there,
I have had the exact same issue - although I have used mine frequently in a live setting and am not the most gentle player.
The local authorised repairer attributed the issue to the poor quality of the instrument - even after a thorough clean out he has not been able to fix it to my satisfaction.

The quality of the instrument is less poor than the repairer said.
Depending on the age, or better the aging (i. e. the weather conditions during gigs, the strength you use while playing, how you store / transport your keyboard and so on) keybed issues can occur to almost every instrument.
The worse thing you may experience is when the plastic junction at the beginning of a key breaks. In this case the key will not return in place after playing, and will even move laterally.
There is no cure for this except getting a spare part (one octave) and replace it. Have a look here
Then there are issues regarding the contacts, i.e. the conductive plastic buttons which close the sensor contacts when you press the key. I had similar issues on a synth action keyboard and after cleaning and graphitizing the non-working contacts I eventually had to get spare parts and replace (one octave at the time also in this case)
Finally, I had the same problem reported here on my MoXF8 last year, after years of intensive gigging (often in bad conditions, e..g. on the seashore), rehearsing etc.
No dynamics problems. no failed notes, no lateral moving, but the keys became "too light" and noisy.
The keyboard was off warranty, so I decided to try to repair it myself before the beginning of the summer gig season.
After opening (see the linked video) it looked clear that dust, hair and so on went in and had to be removed, but not only. The yellow grease put inside the key sliding parts (let's say the key hinge) was degraded and missing in most of the keys. So I thoroughly cleaned everything, and then put inside the right amount of teflon based grease (teflub - this was suggested in an Italian musician forum, and worked perfectly for several users, including myself). Now all keys are like new, they make no huge noise and the key resistance is again like I remember it was when I bought it.
It needed an afternoon of hard work, and a few Euros for buying a grease tube from a famous online marketplace, but it worked.
Hope this helps!!!
P.S. you should also check the felt pads and buttons under the keys. If they are missing ore became too thin, the key will hit the keyboard frame and make noise. Mine were perfect everywhere so I didn't need to perform any action in this field