The following steps recreate the behavior:
Create the text block "Testing font sizes".
Make the word "testing" 24 point.
Make the words "font sizes" 12 point.
Scale the text block 50%.
The entire block becomes 12 point, when only the first portion should be 12 point.
The second portion should be 6 point.
This is specific to text blocks whose first run of text is 24 point. Text blocks scale correctly if the first run of text is other than 24 point.