I do believe that anyone who spend over 180 days in Thailand becomes a tax resident and many people on retirement visas have been asked to go to their Thai bank and clarify their Thai tax status . I wouldn't be surprised if some new tax rules were bought in
Dont you become a Thai tax resident if you stay more than 6 month ? They could easily announce that Thai tax receipts must be shown when entering on DTV
Thais quite frequently change the entry requirements . They could easily change the 6 months DTV entry to 30 day entries, like they are doing with the 60 day visa exempt or require you to have paid tax in Thailand