Thanks FSDG for answering my request:
“Unfortunately, this is a known phenomenon of the MSFS when using addons that contain their own ortho data (aerial images). It looks like the MSFS has received a performance upgrade since one of the last sim updates, which should improve the performance. As a result, large CGL files are not loaded into memory at a certain distance, causing the white areas. Unfortunately there is currently no fix for this. Since the Seychelles are not covered at all or only very poorly covered in the default MSFS, deactivating the CGL file would also make no sense.” (FSDG)