@ericyellin

I upgraded to 10.2.2.

Here's what worked for me:

1. Install CKEditor 4 (If not already installed): composer require 'drupal/ckeditor:^1.0'
2. Disable CKEditor 5 from UI
3. drush cr
4. Checked there were now no errors in Text Formats.
5. Enabled CKEditor 5 in UI
6. drush cr
7. Changed from CKEditor to to CKEditor5 in Text Formats
8. Checked and all looked fine (no errors)
9. Disabled and fully uninstalled CKEditor4 (UI + composer remove drupal/ckeditor)
10. drush cr

Done!

Confirming the issue on Drupal 10.1.1
With the same version (8.x-1.7) on Drupal 9.5.9, it works fine.

My mistake. This is not an issue with the module. Just not clearly documented.
Defining which profiles appear in the registration form is done by adding form display modes to the Account Form Display:
/admin/config/people/accounts/form-display

At the bottom under: "Custom display settings" click the "Manage form modes" to go to the "Forms modes" settings and then under the "User" section create a new form mode for each profile with the profile field you need in your form. These modes will then appear in the Multiple Registration form under the select list: "Form mode to render the Register form".

ericyellin → created an issue.

Closing this, as there are no additional comments.

Thanks for testing and all the insights.
I have decided to move on to what seems like a better solution for serving S3 files.
I have implemented an S3 File Gateway which allows mounting S3 buckets as local volumes on EC2.
This has basically solved all issues since it allows treating S3 buckets as a local FS.
It does require a separate EC2 server to provide the gateway/cache/shares and may not fit for small scale solutions, however since we wish to use S3 storage extensively, this seems like the way to go.

Some more testing.
I opened: https://drupal9-t1.webaxy.com/system/files/2023-01/test1.mp4 on Firefox in a "Private Window" from a remote location.

1. On the Drupal EC2 server I can see two files being downloaded simultaneously each 1.6GB. (in the screenshot you can see them still being downloaded):

2. Once downloaded the video starts streaming, the two first files are deleted from disk and but the stream comes from a third "get" that starts downloading a third file:

You can see from the browser network analysis the time it took the first two files to download (over 21 seconds) and then the video started to stream ("receiving" mode) when the third file starts downloading: