Vault secrets engine partial update resets username_template to default

[skeet70]

Describe the bug
A write to aws/config/root to set the region resets username_template to its default value.

To Reproduce
Steps to reproduce the behavior:

1. Run vault write aws/config/root username_template="{{ printf "vault-prod-%s-%s-%s-%s" (unix_time) (random 20) (.DisplayName) (.PolicyName) | truncate 32 }}", observe that it has been set to the custom value.
2. Run vault write aws/config/root region="us-east-1"
3. Run vault read aws/config/root and see the error that the username_template was set to the default without -prod. Other set values aren't changed.

Expected behavior
The new value was set for region but all other values were consistent.

Environment:

• Vault Server Version (retrieve with vault status): 1.19
• Vault CLI Version (retrieve with vault version): 1.18.2
• Server Operating System/Architecture: Mac OSX Sequoia, Ubuntu

Additional context
The same thing happens when making changes using the UI.