The RadImageEditor control doesn't support the TIFF file format. This is why I have logged a new feature request in our feedback portal. I also updated your Telerik points as a small token of gratitude for bringing this to our attention.
If you insist on using the TIFF format, you can explore the following idea. You can use a third party library that can read tiff files (like LibTiff.Net) this should allow you to read the pixels of the image and convert them to a .NET MAUI ImageSource object (a bitmap image) and provide this to the RadImageEditor.