Yes, I know "ti" is a possible romanization of チ [tɕi] or [tʃi]. The problem is that most people that speak English natively will probably read it as [ti] "tee", not [tʃi] as in cheese or itchy, not only because they will more than likely be unaware of such methods of romanizing Japanese, but because the letter grouping of "ti" in English does not ever, to my knowledge, represent the same sound チ does, which is a sound that exists in English already (or is very close), and "chi" is simply more accurate. You almost never see "ti" used in translated material where a word is left romanized for whatever reason, most often nouns, such as what we are dealing with. This isn't a case of the so-called Japanese l/r ambiguity. We are dealing with a game here, not any context sensitive transcription that may require "ti" to preserve grammar or something within scholarly circles.
For what it's worth, both renderings of ticket exist, as does チケツ, which is not uncommon with loanwords into Japanese, though the others are far less common. チィ is representing a different sound than チ anyway, even if not all speakers make a proper distinction. These combinations, 特殊音 (tokoshuon), were developed relatively recently to better reflect the phonology of foreign words, though there were plenty of words in the lexicon beforehand (not to mention difficulty in pronouncing them), thus the sometimes less-sensical and non-uniform spellings, kind of like how we still say "yen". But as you've said, it's not really the same; チ representing ti is not the same as ti representing チ, simply because English has many more phonemes than Japanese, not to mention sometimes strange spellings for those phonemes. It's inherently different going from one language to the other.
Sure, it may not technically be an error, and if you don't want to change it, then, well, whatever, I guess, but I don't think you've given a particularly good reason as to why it "is fine", even if it's a relatively minor issue.
Anyway, /langnerd.