This usually happens when the sampling width is too wide, causing GIMP to average out too much information. Try reducing the sampling width to focus on the immediate texture around the hole.
The tool in GIMP is a powerful, automated feature used to remove unwanted objects or people from an image by seamlessly blending the surrounding texture into the selected area. Often referred to as GIMP's version of Photoshop's "Content-Aware Fill," it relies on the Resynthesizer plugin to analyze and reconstruct image data. Understanding Heal Selection vs. Heal Tool gimp heal selection
Follow these steps for optimal results: