Well, after fiddling around with the debugger and making various changes to access controls, I've somehow managed to get the system to behave the way I want to. After I managed to get everything working I originally thought it was due to a bad script, but then I compared it to what I put in my original post and it's the same. The way I have it now is that I have inactivated all of the access control rules for specific fields on sys_user, and instead have an update rule which applies on all the fields in the table and which requires admin, and I have another update rule which applies on all the fields in the table and which has the following script (the same as what I had before):

if(gs.getUserID() == current.sys_id)

{

        answer = true;

}

else{

        answer = false;

}

Given that this should be the same state I described above in my original post, I believe that what happened was that something about the system was not actually how I described it before and somehow going through the debugging process and making various changes accordingly just shook it all out. Thanks again for the help. I'll definitely be relying on that debugger if I have ACL issues in the future.