In dispute tribunal we have noun 1 + noun 2 in which noun one acts attributively/adjectivally upon noun 2.
Thus dispute tribunal = a/the tribunal associated with dispute. (Here, "dispute is uncountable, hence singular.) The plural would be "dispute tribunals" - the plural being indicated by the inflection of main noun.
In disputes tribunal we have the same noun 1 + noun 2 - it just happens that noun 1 is plural. BUT because it is acting adjectivally, it does not affect, in number, the main noun "tribunal" - it simply means "a/the tribunal associated with disputes. (Here, "disputes is countable, hence plural.) The plural would be disputes tribunals - the plural being indicated by the inflection of main noun.