Update:

The true ratio of random salts containing the 0x00 byte would be 1 out of every 32.44 messages using an incompatible salt shorter than 8 bytes.

Code:
//echo -a $calc(1/ (1-(255/256)^8) )


The workaround until this is fixed would be to limit the salt to one of the 97% of salt strings which don't contain the 0x00 byte

Code:
alias randsalt returnex $regsubex($str(x,8),/x/g,$chr($rands(1,255)))


then use it with the 's' parameter when creating salted messages

Code:
$encode(message,mcs,key,$randsalt)


After the fix, I estimate that this should decrypt as normal text:

Code:
//echo -a $decode(U2FsdGVkX18AAAEAAOkAAIC4MK5GejRGGZ/W3g4BJoY=,mc,test key)

and
Code:
//echo -a $encode(message,mcs,key1234,5678)

result now: U2FsdGVkX181Njc4AAAAAOZf+ZmPFy3l same as $encode(message,mcs,key,12345678)
after fix:: U2FsdGVkX181Njc4AAAAAKR3f8+Rmn/r