Flyweight Dech, who won a bronze medal in the previous edition in Madrid two years ago, had his campaign ended prematurely in the quarter-finals where he lost to Guillermo Perez of Mexico.
Dech's efforts appeared to land more clearly in the first two rounds, but the Thai only hold a marginal advantage at 4-2.
In the final session, Dech managed to dodge Perez's attempted head kick, which hit his shoulder instead. However, the officials decided to give Perez a point, that helped the Mexican level at 6-6, and the match went into sudden death.
The two remained tentative in the initial phase before Perez attacked But Dech thwarted him with a kick which saw the Thai slip. An official controversially deducted Dech a point to hand the Mexican victory,
It caused a fierce protest from the Thai camp, which saw South Korean coach Choi Young Suk walk into the centre of the ring before security guards intervened.
"I was disappointed with the outcome," team manager Natthapol Teepsuwan said. "I protested because there are two days remaining before the tournament ends. If the decision is not transparent, the athletes will feel downhearted, and not want to compete. I felt very sorry for Dech."
In the welterweight division Dam Srichan, a Muay Thai fighter-turned-taekwondo exponent, also crashed out in the last eight following a 3-0 loss to Sebastian Michaud of Canada, while Laddawan Wachilamekhakul was bundled out of the competition in the first round in the women's flyweight category.