Hi,
Why you are entering payment term directly in MIRO screen!
When you do MIRO for a purchase order, the payment term should flow from purchase order only and you should enter your payment term during purchase order creation.. If you are facing issue with MIRO screen, then use t.code: OB41, Select posting key 31 , Go to details and find the how is the status (Req.entry or Opt.entry or 'Suppress'. ) of payment term
Regards,
Biju K