That coin was minted in 1831 according to the document you linked. That only leaves about 16 years for the coin to make it from India to Utah before the territory was won by or ceded to the United States during the Mexican War around 1847. If it was indeed related to Spanish missionaries it probably would be older than 1831 since those missionaries would've been Mexican. Mexico won independence from Spain in 1821-1822 though the Spanish only officially recognized it in 1836 I think.

It probably found its way there in the pocket of a Mormon who came to California to serve against the Mexicans during that short-lived part of the Mexican War. Or maybe a friar, or miner who got it as payment during some transaction with a Mexican or Spanish sailor in a California port city before the state came under control of the United States. Its placement could be dated more accurately if it were placed in context with the mint marks or dates from coins that it was found with.

There are lots of reasons why people would bury valuables back then and never return to recover them.

My wild guess is someone brought it by sea

Possibly in their pocket! (Did pants have pockets back then?)

Could have been planted by the person who found the cache. People do weird things like that.

I wouldn't call fraud for personal gain weird. It's quite common, actually.

You, sir, just unlocked the case!