Ayaṃ vo amhākaṃ anusāsanī
this is a stock phrase appearing in a number of suttas, just to list a few sn 47.44, sn 43.44, sn 35.146, sn 36.8
some of them start with the phrase
Bhikkhus, I will teach you
but nonetheless in the concluding phrase (this thread subject) plural personal pronoun amhākaṃ is used
in the following phrase the perceived incoherence is even more obvious
“Thus, bhikkhus, I have taught you the destination and the path leading to the destination. Whatever should be done, bhikkhus, by a compassionate teacher out of compassion for his disciples, desiring their welfare, that I have done for you. These are the feet of trees, bhikkhus, these are empty huts. Meditate, bhikkhus, do not be negligent, lest you regret it later. This is our instruction to you.
SN 43.44
in a search a phrase came up, which could hint at one possible reason for this
So conviction & virtue, confidence & Dhamma-vision
should be cultivated by the wise,
remembering the Buddhas’ instruction.
AN 7.7
so it could be that the Buddha speaks on behalf of all the buddhas, as their representative as it were, because their Dhamma is essentially the same
it’s unclear however why he would speak this way and why in the context of those particular discourses
another faint possibility is that it’s a collective voice of the actual composers of those suttas, still the purpose of which is difficult to understand considering the need in maintaining suttas status as purported records of the Buddha’s own words
would be interested in other opinions on the subject