Obviously, it is little bug which won't need any urgent fix.
Auto increment option was not cleared off internally when editing a column which had the
option ticked (with data type eg.int) and subsequently changing the datatype to
non-applicable type (eg. char).
The problem will only manifest when forward engineer the model to script.
How to repeat:
Create a table with a column "whatever" with datatype "int" and make it auto increment.
Change the table by changing its datatype to "char(10)".
Export the model to script
Inspect the generated script and you'll find that the auto-increment is still applied.