Could it be a software bug?
Probably, it's beta software.
Have you raised a Feedback report? If not, please do so at https://feedbackassistant.apple.com/ then post the FB number here.
When you raise the bug, be sure to explain what ringtone you have selected, and any other relevant info such as whether you've assigned a particular ringtone to the person calling you, or whether it's just a general ringtone for all calls. Are you in a focus mode like Mindfulness or Personal, perhaps?
