MySQL Master-Slave Replication additional columns on slave are being reset to their default value
If the table definitions are different, you are asking for trouble and need to be careful.

In particular, if replication is using Row Based Replication, I suspect the missing columns would get defaults, but I would not trust it.

If you are writing to the Slave, you are asking for trouble.

But this may be a workaround -- instead of having extra columns, have an extra table for the extra columns.  The two table would have the the same PRIMARY KEY, except that the extra table would not say AUTO_INCREMENT.