Seemingly invalid sourcemap indexes

[solipsis]

I am trying to tie some sourcemap data to debug transaction traces I am running. I understand the format of the sourcemaps but the values of the sourceIndex seem outside the range I would expect. For example I have the following generated sourcemap (truncated)
"sourceMap": "882:24919:116:-:0;;..." but the sources array in the same artifact only has 38 entries. Way less than the index 116.

Would greatly appreciate any help on what I am misunderstanding here. Thanks
version forge 0.2.0 (ae89c92 2023-09-15T00:16:50.559420910Z)

I have also come across this issue https://github.com/foundry-rs/foundry/issues/1487
Perhaps my issue is that output does not currently include the id field that I need to map on