Similar problem.
The defined variable $wgWhitelistReadRegexp[] = '/^Public:/'; (within LocalSettings.php) works fine when pages are requested with the single URL parameter "title=...".
What happens?:
If the additional URL parameter "action=raw" is used, there is no access to the pages defined by $wgWhitelistReadRegexp.
Example URL that works: https://www.yourwiki.com/index.php?title=Public:YourPageName
Example URL that does not work: https://www.yourwiki.com/index.php?title=Public:YourPageName&action=raw
This is an undesirable behavior.
I would really need public access to some pages (defined by $wgWhitelistReadRegexp[]) including the usage of the URL parameter "action=raw".