yes, airline can ask proof of forward travel if you DON’T have a visa because they’ll get a penalty if they fly you to Thailand and you get refused entry into Thailand.
In case you’re already issued a Thai visa from Thai embassy/consulate airline wouldn’t ask you proof of forward travel.
I would lije to see screenshot of Timatic where it says person with visa issued by Thailand is forbidden to travel with 1 way ticket. I’ve done it bunch of time. Trips from Europe, not one way from Kuala Lumpur or similar