I compared the fastcgi environment variables that Caddy v1 and v2 send over fastcgi, I see no relevant differences.
I went through the thinkphp v5 and v6 framework source code (specifically Request.php, which is where the request is bootstrapped from $_SERVER env vars).
The PATH_INFO var is not needed because there are fallbacks to use REQUEST_URI instead, which should be correctly set. You can probably test this yourself, try dumping $_SERVER in your index.php before the app is booted.
At this point I’m back to my previous answer of “it’s not Caddy, it’s your environment”. There must be something that you didn’t share with us that’s causing your issue.