Something is intercepting bots and loading a different page for them.

I’m facing a very strange issue.
Something on the server is intercepting all web requests from bots that have "bot," "crawler," or "spider" in their user-agent string (there may be others, but these are the ones I've noticed). I spotted this because social sharing links stopped loading correctly, and my already meager AdSense earnings plummeted.
The strangest part is that it isn't a standard block; instead, when a bot is detected, it loads a basic HTML page that says "Welcome Loading..."
I’ve checked everything: there are no filters in Nginx or PHP, and ModSecurity is disabled. I reviewed the Nginx and Apache configuration files and found nothing unusual. I even searched the entire server for that "Welcome" file—or any file containing that text—but couldn't find it.
What could be happening? I would appreciate any help. Thanks.