US citizen with Tier 1 UK visa trying to split time

Hello everyone!
I have US citizenship, a Tier 1 visa for the UK and am allowed to live and work in both countries. My US full time employer allows us to be nomads however they are saying that if I want to split my time 50/50 they need to transfer me to the UK entity and that I can't actually do it 50/50 and I would only be allowed to be in the US for a maximum of 90 days per year. This seems like they are interpreting some law that probably applies to the ‘normal' situation but not my particular one of already having everything I need to be allowed to live and work in both countries.

Anyone know why this might be the case or have a similar situation that they have e been able to find a workaround for? My company is incredibly flexible and are totally fine with me moving around but this just seems like maybe something is getting lost in translation.
You will have to speak to your company directly, and ask them why this is the case. The only thing I can think of that would make it difficult for them, and maybe why they are doing it is because of taxes, and the fact that working in both countries for basically the same country but different branches will cause many tax issues in the UK, and the USA, and they probably don't want to deal with figuring out your taxes.

I can go into tax law all day. But I don't know your situation, and that is something you should speak directly with your company about and a tax expert in the USA, and UK.