We're currently working on fixing an issue where custom workout free ride blocks can set the trainer into SIM mode instead of a flat resistance. This can cause problems if you're entering the block in a gradient section. While we haven't received confirmation that this affects all custom workouts, we recommend using the workaround below just in case.
Before doing a custom workout with a free-ride block, we recommend setting your trainer difficulty to the lowest possible setting (Menu: Settings: Trainer Difficulty: move the slider bar all the way to the left). Even if you do hit the bug with this setting on, you will only feel a flat resistance (which is how the block is supposed to feel).