John P. answered 07/23/21
PhD Student in Linguistics, I tutor Writing, Math, English, Sci, etc.
To my knowledge (note, I am not an expert in historical linguistics, but my PhD advisor is, and I have taken two graduate level courses on the topic), the doubling of the "r" that occurs when going from, for example, "transfer" to "transferring" is a holdover etymological phenomenon from when it was pronounced with second syllable stress. Notably, the standard anglo stress pattern is word initial stress, and a great many words that historically are stressed secondarily have made their way to primary stress at one point or another. For example. the word "nature" used to be pronounced with a clear "t" instead of the "ch" sound, and with stress on the second syllable, but was resyllabified over time after coming into English from French. I agree wholeheartedly that nowadays no one pronounces "transfer" with secondary stress in US English; however, it was almost certainly pronounced that way at one point, and likely for a long time.
The key here is that orthography changes slowly in English, and many words are spelled differently from how they're pronounced because of that. In short, "transferred" is spelled exactly how it's supposed to be given its historical pronunciation, and the issue isn't that it's an exception to English grammar, merely that English orthography is slow to keep up with linguistic evolution.