IMO that is only a problem for those who demand that the issue key needs to go first in the subject (which again IMO is bad for readability). I don't see why you can't just stick it somewhere after all the conventional commits junk? Issue keys ought to be something that can be janked out based on a regex like an alphanumeric prefix followed by a number, so things like this "standard" have little need to set aside a space for it.

Personally (without conventional commits) I tend to put them at the end in parentheses if the commit has something to do with that issue. But if there is a stronger relationship like that it fixes the issue, I put a `Fixes` trailer in the message as well.