Is what the questioner did, visiting his father's grave during another funeral procession, which delayed him slightly from the congregational prayer, a reprehensible act that causes him to miss the reward of accompanying the funeral?

If the brother means that there is no reward at all, then this is incorrect, as the effort of a worker is not lost with Allah, in accordance with His words: "Indeed, I do not allow to be lost the work of [any] worker among you," and "and He will not deprive you of [the reward of] your deeds," and "And whatever good you do - never will it be removed from you." However, if he means not receiving the full reward for accompanying a funeral, then the full reward is specific to one who attends from the beginning of the funeral procession until its burial. But, moving a little away from the group to dig the grave is not considered falling behind, and by doing so, one receives the full reward, in addition to the reward for visiting the father's grave and supplicating for him.