Whether flannel fabric is pre-treated for stain resistance depends on the specific product and manufacturer. Flannel itself is not naturally stain-resistant like some synthetic materials, such as microfiber, which are designed to repel stains more effectively.

However, some flannel upholstery fabrics may undergo additional treatments or have finishes applied to enhance their stain resistance. These treatments can include:

Scotchgard or Fabric Protectants: Some manufacturers or retailers may apply stain-repellent products like Scotchgard to flannel upholstery to make it more resistant to spills and stains.

Nanotechnology Treatments: Some fabrics are treated with nanotechnology-based coatings that create a protective barrier, making it more difficult for liquids and stains to penetrate the fabric.

Performance Fabrics: Some flannel-like fabrics, marketed as performance fabrics, are engineered to be stain-resistant and easy to clean. These fabrics are often designed to repel liquids and resist stains.

Special Finishes: Flannel fabrics with special finishes may have improved stain resistance. 

These finishes can create a barrier on the fabric's surface, making it less porous and more stain-resistant.It's essential to check the product specifications, labels, or descriptions provided by the manufacturer or retailer to determine whether a particular flannel-upholstered item has 

undergone stain-resistant treatments. If stain resistance is a priority for you, inquire about the fabric's stain-repelling capabilities before making a purchase. Keep in mind that even with stain-resistant treatments, it's still advisable to address spills and stains promptly to prevent any potential long-term damage to the fabric.