why does one s3 folder show in my web browser and another folder in the same parent doesn't

I may be how the folder was created. See: Organizing objects in the Amazon S3 console using folders.

Important
When you create a folder in Amazon S3, S3 creates a 0-byte object with a key that's set to the folder name that you provided. For example, if you create a folder named photos in your bucket, the Amazon S3 console creates a 0-byte object with the key photos/. The console creates this object to support the idea of folders.

The Amazon S3 console treats all objects that have a forward slash (/) character as the last (trailing) character in the key name as a folder (for example, examplekeyname/). You can't upload an object that has a key name with a trailing / character by using the Amazon S3 console. However, you can upload objects that are named with a trailing / with the Amazon S3 API by using the AWS CLI, AWS SDKs, or REST API.

An object that is named with a trailing / appears as a folder in the Amazon S3 console. The Amazon S3 console does not display the content and metadata for such an object. When you use the console to copy an object named with a trailing /, a new folder is created in the destination location, but the object's data and metadata are not copied.