IIS RewriteModule与CakePHP表现奇怪

I'm running a CakePHP website as a virtual directory under IIS 8.5, using the FastCGI handler for PHP. I'm usuing the RewriteModule to do the CakePHP rewriting in the virtual directory, which mostly works but there's a beheviour I just don't understand.

If I browse to:

http://mylocalsite/VirtualDirectory/my_things/something

all is fine, the 'something' action of the 'my_things' controller is executed. If I then modify the URL to (for example):

http://mylocalsite/VIRTUALDirectory/some/path

I get a CakePHP error, "VIRTUALDirectoryController could not be found". If I then restart the website in IIS and try again the page mostly loads fine, although sometimes I need to 'Stop' then 'Start' the website in IIS.

I tried using IIS's "Enforce lowercase URLs" rule to force URLs to the same case, this had no effect.

Can anyone help me understand this, and get rid of the weird behaviour?