Of course, that could be due to:

• Non-Browser Clients (Requests from scripts or APIs might not include it.)
• Bots and Crawlers (Many do not send a User-Agent or use generic ones.)
• Privacy Settings (Some users block or modify this header for privacy.)
• Proxies/Firewalls (These might strip certain headers, including User-Agent.)
• Ad Blockers (Some users may have settings that remove or alter it.)

Unfortunately I couldn't find any online docs to share with you about that topic, but these are basic things I knew from years ago, may that help you.