The visitors took the lead early on, but Saints rallied thereafter and drew level via Danny Ings's seventh of the season - only to be stung late on by a Richarlison decider.
“As a group, we're disappointed with how it ended," Scotsman Armstrong said. “We had a poor start and sustained a lot of pressure in the first half.
“We made a few adjustments in the second and got ourselves back into it. There was a positive feeling on the pitch and so to concede again was disappointing.
“It's an upward climb at the end of the game to try and get something out of it and ultimately we missed out.
“When you work hard all week and go into a game thinking positive and wanting to get the best out of it, conceding so early was obviously disappointing."
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