This is a well written document. My only comments are likely due to my lack of familiarity with QUIC.

1) Section 5 states "this frame SHOULD be sent as soon as possible, and MAY be coalesced with other frames." It was not clear to me how this squared with Section 4's "if this bit is set to 0, the Length field is absent and the Datagram Data field extends to the end of the packet." Should the MAY be other packets instead of frames? Or is at most one datagram frame with no length present permitted in a packet, with it being last?

2) Section 3 may benefit from including words a la section 4.6.2 of RFC 9001 regarding resetting state when max_datagram_frame_size is rejected. On first read, it was not clear to me why this value did not latch.