What can cause artesian well water to smell?

Recently, our water from the artesian well started to smell (again). It only smells bad for only few seconds than the smell disappear. We have sulfur-iron removal system, water softener, UV lamp. Only the water from the system I listed above smell, because we also have a Reverse Osmosis installed in the kitchen and the water filtered from it does NOT smell.

We did a hydrocarbon test that was negative. We also did a normal "drinkable water" analysis and it passed with nothing ususual. My question is what are the possibilities of that smell?

PS. I can confirm the smell is not sulfur (different from rotten egg)
See this post for our water results before installing the system if it may be useful

As mentioned in the comments, water is often treated by ozonation in order to sterilize it and ensure it can be drank with no ill effects. Ozone is a very reactive gaseous allotrope of oxygen, produced in situ by exposure of atmospheric oxygen to UV-B light (in fact this happens naturally in the stratosphere, generating a thin ozone blanket covering the world). Ozone can be produced artificially by using a mercury lamp, which emits large amounts of UV light. The water is then brought into contact with the ozone in order to be purified.

When water is collected after being ozonated, typically a small amount of ozone is still dissolved in the water. If the water is allowed to rest for a few minutes after being collected, the residual dissolved ozone diffuses out. Ozone has a very strong, characteristically sharp odour, which can be detected even at very low concentrations. This is likely why the asker's water had an odd smell. Ozone is toxic, though, so exposure should be avoided.

In my opinion, ozone is highly reactive in water forming radicals (like hydroperoxyl which is a precursor to hydrogen peroxide, and other reactive oxygen species, that should reduce odors). Here is a mechanism per a source:

The first reaction that takes place is accelerated ozone decomposition by a type of initiator. This can be an OH-molecule, see reaction 1:

1: $\ce{O3 + OH- -> ?O2- + HO2? }$

This radical has an acid/ base equilibrium of pKa = 4,8. Above this value, this radical no longer splits, because it forms a superoxide radical, see reaction 2:

2: $\ce{HO2? -> ?O2- + H+ }$ (pKa = 4,8)

Radical chain-reaction

Now, a radical chain-reaction takes place, during which OH-radicals are formed. The reaction mechanism is as follows:

3: $\ce{O3 + ?O2- -> ?O3- + O2}$

4: $\ce{?O3- + H+ -> HO3? }$ (PH < ≈ 8)

The OH-radicals that have formed react with ozone according to the following reaction mechanism:

5: $\ce{OH? + O3 -> HO4? }$

6: $\ce{HO4? -> O2 + HO2? }$

During the last reaction, HO2? radicals are formed, which can start the reaction all over again (see reaction 2). As a result, a chain-reaction develops, which is maintained by so-called promotors.

My guess on what is actually occurring is much more subtle. You now have less of something (as a consequence of your water processing) that is instrumental in the transient formation of an H2S smell. The explanation is perhaps reduced nitrate concentration.

Per this source, 'Controlled dosing of nitrate for prevention of H2S in a sewer network and the effects on the subsequent treatment processes', you may now have had a reduced nitrate concentration due to current water processing. The consequence is sporadic regeneration of the sewer smell.

A partial quote from the cited source:

A process for the elimination of septicity based on controlled addition of nitrate to sewer networks is developed...The results showed that dosing of nitrate was very effective for suppression of hydrogen sulphide in a rising main. During nitrate dosing the average level of hydrogen sulphide at the works inlet was 0.24 mg/l, with a daily variation within the range of 0–0.4 mg/l. Without nitrate addition the average background level of hydrogen sulphide was 4.2 mg/l, with a daily variation of 1–10 mg/l. The dosing was accomplished without significant breakthrough of nitrate to the treatment works. Dosing of nitrate resulted in an increased removal of soluble BOD across the rising main, but no significant change in total BOD or COD was registered at the treatment plant during the nitrate dosing. The nitrification process in the biological filters at the treatment plant was improved during the dosing period.