the question you are avoiding to answer is that how are you so sure you are providing 1 signature?

the fact that on top of your screen it says 2of2 doesn't mean you are providing 1 signature. for instance (i just tested this) when you create a 2of2 wallet the first step creates your key, then in second step it asks  you for the second cosigner key. you can provide it with a master public key (xpub....) and NOT be unable to fully sign on this wallet or you can provide it with a master private key (xprv....) and be able to fully sign the transactions since you are holding both keys and providing 2 signatures.

try creating a new wallet but this time in step 2 make sure that you are giving it a string that starts with the word "xpub". if you still could sign transactions then we can start digging more to see what the problem is.