Buyer pulled out saying my leasehold flat is unmortgageable due to stepped ground rent

TLDR: Buyers pulled out last minute saying my leasehold flat was unmortgageable because it has stepped ground rent, but everyone I’ve spoken to is giving different advice.

Since then I’ve spoken to the estate agent, my solicitor, the freeholder and a mortgage broker, and everyone seems to be saying something slightly different.

Leasehold flat (2 floor building) in England. Lease is 125 years from Jan 2008, so around 106 years left.

Ground rent is £50 per year now, then goes to:

£400 after 25 years

£800 after 50 years

£1,200 after 75 years

£1,500 after 100 years

So it goes to £400 in around 2033.

Is this actually likely to make the flat hard to mortgage, or are the buyers possibly just using it as an excuse?

Freeholder has suggested looking at a lease extension/renewal to peppercorn ground rent, but I’m trying to work out if that’s genuinely needed before selling.

Has anyone dealt with anything similar recently?

Thanks for any and all help, I'm praying this dosent cost me thousands of pounds