People dont WANT to you to find them by phone number. Its not a bug and its not happening randomly, people are just enabling the very good setting that prevents others from finding them by their phone number.

This has literally zero correlation with whether you can search your contacts by phone number, it is in fact the exact opposite

This is creating a signal account that has no phone number associated with it.

From the article:

"The ability to register for Signal without a phone number is a frequently requested feature. While Signal hides phone numbers by default and gives users control over who can discover them, some users would still like the option to create an account without using a phone number."

They added a username feature a long time ago that makes finding each other very reliable.