Thailand does have low inflation.
Also, although some always object, pricing is usually a function of supply and demand.
On the demand side, the farang customer base has had to contend with a strong baht. For example, it's way easier to be generous at 70 baht to the pound than 40 baht. I suspect a flat exchange rate would have led to a faster rise in tips.
On the supply side, the proportion of Isaan boys working seems to be decreasing and their place is taken by lads from Lao, Cambodia, Myanmar and even Vietnam. The 1200 baht tip probably doesn't have the attraction that it once had for a Thai, but might look good compared with a regular job in Laos.
I could do a Bangkok poll, but am not sure what participation rate we will get.
Another interesting point would be any trends in tip dispersion.
Are the 1000 baht tips paid typically to twinks, phone app boys and those from "cheaper" bars ? (Sunee, Jomtien)
Are the 1500 baht tips paid to muscle boys, and boys from more expensive bars (Toy Boys, BBB) ? Or is it just more generous customers?