I recently replaced my under sink RO system with a new one. On filling a glass with water
There is an initial bad smell (drainlike) from the water. After a few seconds it dissipates.
The water from the regular faucets is o.k.

I called the manufacturers of the RO unit and they told me that it is a water supply problem as my water contains gas. I then called the water company who told me that gas in the water
Is an impossibility. I'm inclined to think that the RO company is giving me a snow job because the smell is isolated to water coming from the RO storage tank. Anyone come across this situation before?

Call the RO company and demand a replacement. Gas in the water, huh? Well, at least they were original. They were thinking hydrogen sulfate which smells like rotten eggs but, as you've pointed out, it only comes after the RO.
My guess is the RO was shipped with bacteria already in the system. Regards, Tom

Think I have solved the problem. I drained the tank and put some chlorine in one of the filter
Bowls after removing the filters and let the tank fill and sit awhile then drained it put the filters back and refilled. So far the smell has gone.

It is obvious that the Watts Premier Co, were giving me a snow job. Luckily the contaminated water does not seem to have caused any health problems.