Why Are Images Scrambled When Uploaded to S3 by Different Users?

I think the core of your problem may be that your code is writing to a temp directory that’s shared between requests. If two functions run back-to-back, they might overwrite each other's files. Try creating unique temp paths for each user’s upload by appending random strings to the filenaming scheme.

When you're dealing with multiple users' uploads, it's crucial to ensure there's no overlapping state in your Lambda functions. You might want to consider implementing more unique identifiers in your temp paths to prevent image scramble.

Have you confirmed the signed URLs generated for both users? If there’s a mix-up in those, it could also lead to unexpected behavior. Also, if you suspect caching, adding some debug logs will help identify the root cause.

It sounds like you might be just caching a key and inadvertently using it on a different request in your Lambda function. Adding some logs to trace the process might help clear things up.

Are you sure the Lambda functions are completely isolated? Sometimes, Lambdas can warm start, meaning if there's any data cached in their temporary storage, it could lead to the old image being fetched again. Check your code to see if you're saving images in a shared temp directory without unique identifiers.