Summary
There is an invalid if condition when checking if a file exists in the cache, in the S3 storage source extension, which causes the exist variable to hold the wrong value. This bug was introduced in WT-8712, when refactoring some of the code.

Suggested Solution
In ext/storage_sources/s3_store/s3_storage_source.cpp on line 101, change:

if (*exist = !S3CacheExists(fileSystem, name))

to

if (*exist = S3CacheExists(fileSystem, name) == false)