Interesting. I am able to reproduce what you are seeing, more or less. (I think I was getting FILLET less than half of the time.) FASTSEL is an Express Tool (and set up to respond to FS as well).
This appears to be an effect of the AutoComplete function for Command Line input. If you disable that, and hit the spacebar or enter key before the Command Line Suggestion List deploys (before the INPUTSEARCHDELAY time expires - default is 300 milliseconds), the command alias works without fail. Of course, with AutoComplete disabled, if you leave the other options for the Command Line Suggestion List active and fail to execute the command before the list deploys, you will not have the FILLET command on the list of choices. You could disable all of the Command Line Suggestion List features so that the list never shows up, but I find that useful and would not want to do that.
OK - here is something of a workaround. Left click on the wrench icon at the left side of the Command line, and in the Input Search Options dialog, verify that you have Sort suggestions set to "According to frequency of use" rather than "Alphabetically". Assuming you use FILLET more that FASTSEL, FILLET should eventually rise above FASTSEL or FS and become the default AutoComplete choice. Unfortunately, it appears that the initial sorting is alphabetically, until AutoCAD has a chance to learn what you use the most. Even more unfortunate, with AutoComplete active, it was running FASTSEL more that FILLET for me, dooming any hope of FILLET rising to the top. I did find that if I disabled AutoComplete temporarily and then used the FILLET command (via the "F" alias) many times, that I was able to get FILLET to the top of the list and now my "F" alias works, even with AutoComplete turned on. Provided that FILLET remains your favorite command that starts with "F", that should stay at the top and the alias should be respected.
David Koch
AutoCAD Architecture and Revit User
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