I beleive i have a similiar ( if not same ) scenario as you.
I have an HD account with mycelium but also have a Single Address account in which the Single Address has the BTC in it.
( When my funds were in the HD Account it kept Saying backup funds, and after numerous attempts at fixing this , the only resolve i foind was moving them to the Single Address and backing it up .Says local trader account in green.

So i Followed the Same Directions that worked for everyone else / Put in my 12 word passphrase from Mycelium and it Transferred My HD Accounts Address with No balance , and My Single Address Remained.

So im not sure HOW to Transfer the Single Address bc there "Apparently" isnt a 12 word passphrase connected to it??

The BTC has been in this Single Address well Before August .

Im not Sure How to transfer funds to my Original HD Account, (i could figure that out ) but not Sure i should since the " snapshot" was taken while the BTC was in Single Address.

So i try to follow what you did and trasfer to HD Account ( that i already copied to Coinomi) ...
OR i try to Sweep My Single Address into Coinomi like @libert suggested ( also don't know how to go about doing ) lol.. Laughing out of frustration but in HOPES SOMEONE CAN GIVE SOME HELP.

THANK ANYONE IN ADVANCE.