I found this post when I was searching for the same issue and guess what I did resolve checksum issue I was facing. I can see your issue is also same

Group 1 =
448=FIO1CHULC69 447=N 452=17
Group 2 =
448=XOFF

There are 2 groups of party ids but in the tag 453 =1 , may be because 448 is lonely it supporting siblings are not present , it should be 453=2 . This is why checksum calculated by FIX is always +1 because value actually the group has 2 element fields

Either need to remove the lonely 448=XOFF or need to update 453=2 and add the supporting field 447 and 452 to it.