Wrong length of leave on dependant visa

[Nifemy]

Hello good morning guys
Please can someone advise me on what to do
I got a 3yr skilled worker visa last month and my dependent applied from outside the Uk
He got to the UK about 3 weeks ago and the post office said his BRP is not with them
We sent an inquiry to the home office and they said he does not have a BRP because his length of leave is less than 6months
Please how is that possible when I have 3yrs visa
Please what do you advise we do

[Frontier Mole]

What does the letter granting the dependent visa state - is there mention of the visa type and expiry?
What does the vignette state - skilled work dependent?