Sometimes we receive incorrect player position information from our data provider. You can correct the issue if you download the player list for your slate from DraftKings or FanDuel and read it in with the Use DKSalaries.CSV or Use Lineup CSV button.
If you are playing in multiple slates, or leaving the Optimizer in your browser across an update, it can sometimes get confused and still get the position wrong. If this happens just reload the optimizer.
Also, load the player list first before generating a lineup. This helps it not to get confused.
We are working on resolving this, but hopefully you can avoid the issue for now.