A PDF with my comments inline is on my OneDrive share at:
https://onedrive.live.com/?cid=DC2B364F3F06FEA8&id=DC2B364F3F06FEA8%21522413&parId=DC2B364F3F06FEA8%21522414&o=OneUp

Summary:
* Overall the document is well written and readable
* Section 2.3 has a nice list of acronyms but many acronyms are in the document that are not listed in that section
* Many typos and grammatical errors exist, as noted throughout my marked up copy
* It is unclear whether a prefix can be a multicast prefix or not. For example, in Table 1 in the discussion of updating RFC 8505, the table has separate values for unicast, anycast, and multicast, but only one value for prefix.
* There are a number of uses of "should" that would be better as "SHOULD" or "MUST" depending on the intent.
* Section 7.3's use of a 112 bit prefix is confusing given that the field length is 120 bits not 112 bits.  Where does the number 112 come from?
* Section 7.4 talks about the case where "it determined that the 6LBR is legacy and does not support this specification" but it does not explain how a 6LR can determine this.  Elaborate.
* draft-ietf-6lo-multicast-registration is used normatively so should be a Normative, not Informative, reference