Reproducing Steps:

1. type "size 11" in a blank cell of a blank sheet
2. change its font size to 18
3. press ctrl+z to undo

Actual:
font actually revert back.
in dropdown box, it still display 18 instead of 11, we cannot set font size to 11

Expected:
font size dropdown displays 11.

Work-around:
change back to another size (10) then change to 11"