It only allows you to go to 250 the other 50 comes from another aa for each tradeskill I think anyway. Look at the first few levels of tailoring mastery and see if they don't give the first 50 than reduce failures and last few levels take it to 350.
I have had this aa for so many years I couldn't remember. Has it been wrong with the text this entire time?
Yup. It's even been reported and confirmed here https://forums.daybreakgames.com/eq/index.php?threads/new-tanaan-crafting-mastery-aa.260556/ 3 patches ago and never fixed.