Byron made it 1-0 at 7:13 of the first period. He got behind Blues defenseman Robert Bortuzzo and redirected Artturi Lehkonen's pass for his first goal in six games since returning from offseason hip surgery.
Thomas tied it at 1-1 at 18:26 on a 2-on-1 with Buchnevich while shorthanded.
Each team had opportunities in the third period. Canadiens defenseman Jeff Petry's wrist shot from the point hit the right post at 7:36; Thomas hit the right post at 12:45 during St. Louis' second power play of the game; Montreal forward Laurent Dauphin was stopped on a shorthanded breakaway at 15:34.
Buchnevich gave St. Louis a 2-1 lead at 18:41 when he scored on a one-timer from the right circle.
"That's a frustrating one because we need every point we can get right now," Blues forward Ryan O'Reilly said.
NOTES:Caufield's first goal was Montreal's latest game-tying goal since March 15, 2014, when David Desharnais scored at 19:59 of the third period. Desharnais assisted on the winning goal by Francis Bouillon in overtime, a 5-4 win against the Ottawa Senators. … Caufield is the fifth NHL player to score in overtime after scoring a game-tying goal in the final 10 seconds, joining Mikael Granlund (Nashville Predators, Feb. 27, 2020), Marian Gaborik (Los Angeles Kings, May 3, 2014), Marco Sturm (San Jose Sharks, Nov. 8, 2000) and Neal Broten (Minnesota North Stars, Nov. 26, 1983). … Canadiens forward Joel Armia left the game at 6:26 of the third period after he was struck and cut by Thomas' skate. St. Louis said he seemed fine but would be reevaluated Friday. … Blues defenseman Justin Faulk extended his point streak to five games (one goal, four assists). … Buchnevich has one goal and six assists during a four-game point streak.