Your question unfortunately presents very little detail to understand what the actual problem is. I would guess it has something to do with the data types of the variables passed to mixed. Like most R modeling functions it behaves differently for factors versus numerical variables. You can check the type of your data with str(), e.g., str(my_data).
Also, mixed should behave completely identical for data.frame and tbl_df as it converts the data to a data.frame (see line 164 at: https://github.com/singmann/afex/blob/master/R/mixed.R). If your could show that this is not the case, this would be a bug.
If you want a more precise answer, provide a reproducible example: http://stackoverflow.com/q/5963269/289572