Some people seem to mistake the community forums as a place to contact Monzo and receive customer support. Quite unsurprising for those who are tech averse trying to navigate Monzo’s help section which may or may not be lacking a chat with us button.
With that said, it’s not terribly difficult to code a form that can allow for format variations and correct for them. I always thought Monzo did this, but if not, or if it’s broken, it needs fixing/implementing. In the meantime, try @Ordog’s suggestion.
I have worked with systems in the past where giffgaff numbers would just not register with us as a valid number for VOIP forwarding because the prefix was missing from our database. That may also be the case here, which would require someone at Monzo to fix. It’s a 2 minute job.