I have spent the afternoon attempting to narrow this down - I tested with 7.4.8 and even 7.4.6 - and the problem persists.

I then took WordPress out of the picture, and just used the single-script OpCache GUI (https://github.com/amnuts/opcache-gui/releases/). That script will load ONCE, display valid figures and after it ran once, FastCGI will fail on page refresh.

I have attempted adding a Cache_ID, using the CWD option, setting up a different IIS application pool just for that one test site. Nothing makes a difference. The ONLY two things that work are - switch that site to run through PHP 7.3.21, or disable opcache.  Only then will the OpCache GUI project and/or WordPress 5.5 run.

Right now the problem looks like a general/generic problem with OpCache under 7.4 on IIS FCGI, but I realize that this is not a likely scenario to not have long since been encountered.

I can not reproduce this with opcache-gui-2.5.4 (haven't tried
with Wordpress).  Could you please provide a minimal php.ini with
which you can reproduce the issue (i.e. start with an empty
php.ini, and just add the necessary settings).  Also, does the
OPcache error log provide any insights (opcache.error_log set, and
opcache.log_verbosity_level=4)?

As requested, I've set up a fresh PHP 7.4.9 instance, empty PHP.ini, and a fresh web site. 

Along the way I stumbled across more places where classes supposedly don't exist and after a day of banging my head against walls everywhere, was able to extract it down to one simple/mininal case -- with huge impact. 

It appears that any subclasses that are defined BEFORE the parent class could be referenced in code prior to the class definitions under 7.3.21 - but will NOW FAIL as non-existing in 7.4.9.

I don't know yet, if THIS is the root cause - but I'll report the following separately as a 7.4.9 problem. I'll keep testing... but this may not be an OpCache-specific problem:

<?php
declare(strict_types=1);

var_dump( class_exists( 'MySubclass1' ), class_exists( 'MySubclass2' ), class_exists( 'MyAbstract' )  );	// PHP 7.4.9 = FALSE, true, true (PHP 7.3.21 = TRUE, true, true)

class MySubclass1 extends MyAbstract{}
abstract class MyAbstract{}
class MySubclass2 extends MyAbstract{}

var_dump( class_exists( 'MySubclass1' ), class_exists( 'MySubclass2' ), class_exists( 'MyAbstract' )  );	// true, true, true
    
// Produced with empty php.ini !

THIS seems to be OPCACHE clashing with UOPZ (the June 2020 release) - if BOTH are active under Win x64 IIS FastCGI, then out-of-the-box WordPress will fail after refreshing home page thrice.

extension=uopz
zend_extension=php_opcache.dll

If EITHER extension is removed, then all will function.
OR, if stepping back to PHP 7.3.21, then both extensions will happily co-exist.

> THIS seems to be OPCACHE clashing with UOPZ (the June 2020
> release) […]

Thanks for tracking this down!  I can confirm the incompatibility,
but since it's an uopz issue, I'm closing this ticket in favor of
<https://github.com/krakjoe/uopz/issues/129>.