AFAIK - The Label Tool text does not have the "fixed or scaled" option like the Text Tool.
This has been previously posted on the AC Wish List.
It looks like you found another mis-documented feature that was never implemented.
David Larrew, AIA, GDLA, GSRC
Architectural Technology Specialist
a r c h i S O L U T I O N S
WIN7-10/ OSX 10.15.7
AC 5.1-25 USA