Any chance you can post the tif file somewhere so I can test and tell you what’s going on ?
Also, beware that the inpainting filter in the G’MIC plugin for GIMP does things a bit more complicated than just invoking command -inpaint. You can get the exact command used by the plug-in by selecting Output messages -> Verbose [layer name], apply the filter then look at the layer name to get the exact command invoked.