Repeat Grids and Symbols in XD both support "overrides," where you can edit text or images in just one grid cell, or one copy of a symbol — allowing it to differ from the others.
When you nest Repeat Grids and Symbols, the innermost one is the 'scope' for the overriding. So in your example, a Repeat Grid inside a Symbol, the image override will make one grid cell different from the others, but the overall Repeat Grid (collection of all cells) will still be identical across all copies of the outer Symbol that contains it.
One way you can get around this is to use a grid of Symbols instead of a Repeat Grid — i.e. a bunch of copies of the same symbol, laid out by hand. Unlike a Repeat Grid you won't be able to drag in a stack of images to automatically spread them across those symbol copies, but you will be able to make each copy of the inner symbol different without those changes syncing to other copies of the outer symbol.
Hope that helps! Let me know if this answer needs some diagrams or illustrations to be more understandable :-)