I watched the reality show Masterchef, and Gordon Ramsey said “I believe congratulations IS in order”. He’s British.
If you google this, it seems that “Congratulations are in order” is the far most common way of saying this (i.e treating the noun as a plural, rather than a singular seems to be the standard.)
I think the “congratulations is in order” interpretations come from treating the noun as a citation:
“I think a ‘CONGRATULATIONS’ is in order”.
In general, citations wreck all kinds of havock to the morphology and syntax of sentences!