Your claim is right and I also think you should be paid in full. The casino should have adjusted your stake against the max win but instead it didn't and hence should pay the full amount since they accepted the bet.
I have removed promotional materials supporting them for now. Hope they will hear you out and will pay you in full.
I have spoken with the marketing agent many times and they have always been very nice and easy to deal with. I also understand they are in the midst of fixing these errors, but at the time of the bet the errors were not fixed. Was definitely not me exploiting them in any way. I just happened to misclick, their casino accepted the bet, and I won. Should be a no brainer to pay the bet and move on, rush your devs to get things fixed faster.