The deposed table-toppers were a point clear at the summit heading into Wednesday's fixtures, but surrendered a one-goal advantage after Nurul Sriyankem had given them the lead shortly after the hour mark.
Jitpanya Tisut netted the leveller after 73 minutes to earn Chainat a valuable point in their fight against relegation. Jadet Meelarp's men now sit one point clear of the drop zone ahead of Sunday's trip to Air Force Central.
Chonburi's slip-up opened the door for reigning champions Buriram to regain top spot having slipped to second a week ago.
Javier Patino put Buriram ahead inside the opening 10 minutes at TOT, before the dismissal of home defender Noppon Pol-udom boosted their cause.
Carmelo Gonzalez made sure of the points in the 73rd minute to put Buriram in the driving seat ahead of their final fixture - at home to Police United at the weekend.
Third-placed Muang Thong United's slim title hopes had already been hit by a nine-point deduction for crowd trouble during a recent clash with Singhtarua, but they remained in good form on Wednesday by extending their unbeaten run to five with a 2-0 win at Osotspa.
The sanction - scheduled to be applied following the final round of fixtures - has plunged Singhtarua into a relegation battle, but a 3-2 win at Bangkok Glass, which included a last-minute winner from Predrag Sikimic, sees them effectively outside the relegation zone on goal difference.
When Singhtarua's deduction - which has been appealed by both affected clubs - is taken into account, PTT Rayong are three points adrift of safety following a 2-1 defeat at BEC Tero Sasana.
A succeSsful appeal by Singhtarua would relegate Rayong regardless of their final result on Sunday.
Police United's three-match winning run has seen them edge closer to survival, with a 3-1 win over Suphanburi their latest triumph, while Sisaket are safe following a 4-2 win against Army United.
Samut Songkhram are already assured of finishing bottom, but claimed a 2-1 win at Songkhla United - just their second victory in 20 league fixtures.
Elsewhere, Bangkok United defeated Air Force Central 3-0, and Chiangrai United held Ratchaburi 2-2.
