Pension lump sum and RMD

I am getting a lump sum of $697,000 from my pension plan but they want to declare $53,709 as ineligible to

roll over and would require me to pay taxes on this amount . I have been taking regular distributions from this 

pension since 2020 at a $5,900 per month . Why the RMD applied to this .  Should be covered on my monthy withdrawals ? can you explain    [email address removed]