A backing or membrane can have different heat limits from the fabric on top. A washable outer layer does not establish that the complete protector can be tumble dried.
Understanding this situation
Waterproof sheets and protectors can also fold into forms that affect movement. The exact garment and appliance instructions are needed; a generic cotton setting may be unsuitable.
A useful next step
Use the prescribed drying method and load limits. If backing peels, melts or cracks, stop using the damaged item and report any contamination of the dryer. Do not try a cooler trial on already compromised material to find a workaround.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.