Your frames are in "combine" mode, so the most likely explanation is that your GIF is "optimized": a frame only contains the pixels that are different from those in the previous frame, and everything else in the frame is transparent. You can restore complete frames (if fact you must, if you intend to edit anything) using
Filters ➤ Animation ➤ Unoptimize. You can later re-optimize the GIF using
Filters ➤ Animation ➤ Optimize (for GIF) before exporting your final result.
Caveat: your image is large and you have many frames, so expect long processing times and intense disk activity unless you have a lot of RAM. To make sure that Gimp uses as much RAM as possible, see
Edit ➤ Preferences ➤ System resources ➤ Resource consumption ➤ Tile cache size and set it to your RAM size minus your floor RAM usage (OS+base apps).