Cushion Floor Discolouration.

If not sealed, why not peel it back and inspect underneath?You would soon see if the cushion layer has got wet as it may still be damp, or just discoloured

Turmeric is notorious for staining any surface. Have you recently cooked with this spice and dropped a smidgen ?

PS did the firm apply a waterproof seal all around the edges?
I imagine not as vinyl flooring expands and needs to move

I can imagine that in a lobby where the vinyl flooring is narrow, maybe only three feet wide or so, it would be easy for water to get under the edge and pool somewhere

PS my vinyl floor isn't Cushionfloor as I opted for hard wear layer instead of a cushioned layer. Therefor I think your discoloration comes from the foam cushion layer not the vinyl layer.Did you have spill that my have made the dye from the cushion foam come out?

I have the same sheet of vinyl on my living room floor for about twenty five years and never had a problem of discoloration. That being said, I never covered it up with a mat. My theory is it's so easy to clean why would I want a mat.

I had this problem, in a previous house. Solid vinyl laid on a concrete kitchen floor. It turned out that it was the pipes taking the hot water from the boiler causing the discolouration.

Did it come with a guarantee? Can you get the company to come back out?
Also, do you know the brand?
If no luck with the floor people, ring the company that has made your flooring.
Maybe you paid on a credit card?
You would expect 5yrs plus for flooring to look good. You sound very careful, mats, etc.
Carport should offer protection from the sun. (I know you thought it could be a shadow).
Good luck with that 👍🏻