It's not an issue with the Flow, but the fact that updating the user's profile picture doesn't run any triggers for User. The same applies to the banner photo.
You can quickly test this with apex triggers and debug logs (none will be generated). You'll also note the
LastModifiedDate does not change if you just change the photo.
In terms of documentation, it's not explicitly called out (though others have noted this behavior) but I believe the issue is due to the fact that the fields on User that correspond to the User photo are derived fields from the Photo record (prefix: 729) that contains the image. Similar to how a formula field wouldn't trigger an update on a given object if its related object value changes.
You see mention of this in the Change Event Message Structure
The fields that a change event doesn’t include are:
- The IsDeleted system field.
- The SystemModStamp system field.
- Any field
whose value isn’t on the record and is derived from another record or
from a formula, except roll-up summary fields, which are included.
Examples are formula fields. Examples of fields with derived values
include LastActivityDate and PhotoUrl