Reddit mentions of The Bhagats of the Guru Granth Sahib: Sikh Self-Definition and the Bhagat Bani

My understanding is that these are not exceptions to the "rule". The Gurus are not following the literary style of the bhagats - rather Guru Arjan and Guru Amardas are providing their commentary to the thoughts first expressed by other bhagats including Kabir and Farid. These are meant to clarify the thoughts expressed by the other bhagats so as to make the bay consistent and avoid any misinterpretations. In doing so, they rightly chose to keep the moniker associated with the bhagats themselves.

For more, read Bhagats of the Guru Granth Sahib Bhagat Poets of Sri Guru Granth Sahib by Jhalman Singh Gosal.