"As might be expected, Maugham was particularly scornful of the lessons in French, taught by masters who although they had a good grasp of grammar made no attempt to reproduce what they considered an entirely unnecessary foreign accent."
Please pardon my ignorance. Doesn't the clause "although had a good grasp of grammar" refer to the subject in the main clause, i.e., "masters". Hence, it does not require "they".
(And, by the bye, is "although.... grammar" called "non-defining clause"?)