English proverb for "They danced, but didn't take a bow", as for failing good work on a final step

Solution 1:

There is a phrase that has been adopted from gymnastics stick the landing, meaning to hold perfect form in the final jump or dismount.

The term has been adopted into broader usage

Execute flawlessly from the beginning through the end. Follow through. All phases of the sales cycle require great attention to detail but to be successful, we must "stick the landing" to close the deal.

(The Urban Dictionary)

The question is about the antithesis. You could say

He gave quite a performance, but ultimately failed to stick the landing.

Or you could turn around a well known aphorism and say

He snatched defeat from the jaws of victory

The original, is attributed to US Congressional Representative James Seddon of Virginia for claiming that a regiment in the Mexican-American war had "snatched victory from the jaws of defeat."

Solution 2:

They fell at the last hurdle¹ would be appropriate if that final shortcoming meant the results were completely worthless. In other contexts, Close, but no cigar!² might be better.

EDIT: Since no-one else seems to have mentioned it, there's also fail at the last hurdle, with an estimated 114 instances in print (vs 410 for fall at the last hurdle, 264 for fall at the final hurdle).

¹ yourdictionary

² thefreedictionary

Solution 3:

You could paraphrase T S Eliot’s well-known final line from 'The Hollow Men' with something like: “[They/He/She/It/What] started with a bang, {but} ended with a whimper."

(example of usage from ‘Stick with It: How to Overcome the Obstacles that Keep You from Following ...’ via Google Books)

Or for a less literary option, you could borrow a phrase from “The King of Sports” (perhaps better known as "The Sport of Kings"): “They/She/He/It started strong/set the pace, but faded in the {home} stretch.”

(example of usage from ‘The First War on Terrorism: Counter-terrorism Policy during the Reagan …’ via Google Books)