EUSS Family Permit problem

Hi guys gals,

I'm asking on behalf of my parents. My mother is a Romanian citizens with EU pre-settled status currently in the UK. My father is non-EU currently in The Rep of Moldova. My parents are married.

Back in May 2020 they applied for an EU Settlement Scheme Family Permit for my father and due to covid related delays he was finally due to fly to the UK today, but was denied boarding. The reason given is that his sticker says "EU SETTLEMENT SCHEME FAMILY PERMIT to acc" and apparently the last 2 words mean my mother must accompany my father on their travels. It has been a while since they filled in the application and at the time they did plan to travel together, but everything changed with Covid restrictions. I didn't manage to find any information on the internet that there are 2 subtypes of this permit, but I did find an image of somebody else's permit that said "D - EU SETTLEMENT SCHEME FAMILY PERMIT to join". I have no idea what the letter D means in this wording, but note the "to join" bit. Can anyone advise if my father was correctly denied boarding or if it is an error how can I get the Home Office to assist? Any other advice would be greatly appreciated.

P.S.
I already spoke to the EUSS resolution center, but they can't help as they don't deal with these visas. The UKVI phone line isn't open till Monday, but I highly doubt they will be able to do anything.