Feel for you
@Mr H. I had a similar experience which ended up in my favour simply because I managed to screenshot the entire app which I don't normally do but this particular time I figured if I did so I'd have a handy reference for later applications.
I'd applied for a QFF Premier card from Citi and they had changed the offer after I applied to a lower bonus and one that was split by a year to retain customers for longer.
The onus was on me to prove it apparently (despite their having my date of application) but my screenshots had all the detail and not only did I get my full bonus, their system paid me the 12 month bonus too (the CSM tipped me off that this would likely happen so I decided to keep it)
I get this probably feels like rubbing salt into the wound but I was lucky in this instance and I remind myself to do the same going forward because Citi were absolutely not suggesting they would have helped me out otherwise