Setting aside whether the original letter was actually capitalized, the rule is common in English writing: https://style.mla.org/capitalizing-start-of-quotation/ ("If the first letter of the first word you quote is capitalized in your source, use a lowercase letter enclosed in square brackets").
More specifically, The Bluebook: A Uniform System of Citation provides one of many sources of angst for first-year law school students in the United States. Its secondary purpose is to prescribe much of the structure and formatting of U.S. legal writing, down to whether there's a space between the period and the capital S in something like "314 F. Supp. 1217 (N.D. Tex. 1970)" (there is), and whether there is one between the period and the 3 in "945 F.3d 265 (5th Cir. 2019)" (there isn't).
The most important Bluebook rule when altering quotations is not to change the meaning of the original text. The second most important rule is to explain the purpose of each alteration through a long set of rules (whether emphasis was in the original, whether a typo existed in the original, etc.). That covers your concern.
More specifically, The Bluebook: A Uniform System of Citation provides one of many sources of angst for first-year law school students in the United States. Its secondary purpose is to prescribe much of the structure and formatting of U.S. legal writing, down to whether there's a space between the period and the capital S in something like "314 F. Supp. 1217 (N.D. Tex. 1970)" (there is), and whether there is one between the period and the 3 in "945 F.3d 265 (5th Cir. 2019)" (there isn't).
The most important Bluebook rule when altering quotations is not to change the meaning of the original text. The second most important rule is to explain the purpose of each alteration through a long set of rules (whether emphasis was in the original, whether a typo existed in the original, etc.). That covers your concern.