Is this an exception to the rule that thật usually goes before the adjective?
Yes it is an exception. "thật" still plays as an adverb equivalent to "very", but a lesser degree so it's best to use "really". "thật" at the end of the sentence also attaches an emotion with what it's indicating (could be positive, negative, sarcastic...) (e.g. ngu thật, đau thật, chán thật, vui thât...)