Minesh asked me to look through this thread and step in, and I'm not sure how much help we can be here in support.
It seems you had a problem with the notifications when using SMTP plugins, which you resolved somewhat serendipitously by changing the sender email details, but which continued to work after reverting the sender email details. So currently things work, even with a different email sender (if I understand you correctly).
We don't really have the resources to speculatively try to break the functionality. If we have reports that something is broken, then we'll work to identify the issue and fix it (ourselves, or by passing it on to the developers). So if we get more reports that a non-default sender email breaks the notifications we will work on them as and when we receive them.
There have been quite a few issues with notifications in the past, but we don't have any currently open issues. The latest update (to Forms 2.6.13) helped clear up some problems that had been lurking since WordPress 5.5, and hopefully they will be more stable now.