Docker PHP-Apache intermitent errors - php

Question

I'm learning about auto-loading by following a simple PHP tutorial. The code of the classes and the custom autoloader do not seem to be the issue.
But, when it makes no sense at all, I get . . . failed to open stream: No such file or directory in . . . errors.
Then I happened to notice when I refresh my browser a few times the expected correct output does display.
So, it's not working one moment and then it is, after a few refreshes. I'm guessing it has something to do with the server. But how can I diagnose this issue ?
I am using the following docker-compose file:
version: "3"
services:
www:
image: "php:7.4.24-apache"
restart: 'always'
ports:
- '81:80'
volumes:
- ".:/var/www/html"
UPDATE : Docker log output
docker logs --follow complete-object-orient-php-developer-course_www_1 | grep Autoloading
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 172.21.0.2. Set the 'ServerName' directive globally to suppress this message
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 172.21.0.2. Set the 'ServerName' directive globally to suppress this message
[Tue Oct 05 02:33:20.966439 2021] [mpm_prefork:notice] [pid 1] AH00163: Apache/2.4.48 (Debian) PHP/7.4.24 configured -- resuming normal operations
[Tue Oct 05 02:33:20.966629 2021] [core:notice] [pid 1] AH00094: Command line: 'apache2 -D FOREGROUND'
[Tue Oct 05 02:33:25.797390 2021] [mpm_prefork:notice] [pid 1] AH00170: caught SIGWINCH, shutting down gracefully
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 172.21.0.2. Set the 'ServerName' directive globally to suppress this message
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 172.21.0.2. Set the 'ServerName' directive globally to suppress this message
[Tue Oct 05 02:33:32.385570 2021] [mpm_prefork:notice] [pid 1] AH00163: Apache/2.4.48 (Debian) PHP/7.4.24 configured -- resuming normal operations
[Tue Oct 05 02:33:32.385663 2021] [core:notice] [pid 1] AH00094: Command line: 'apache2 -D FOREGROUND'
[Wed Oct 06 01:34:55.303031 2021] [php7:error] [pid 20] [client 172.21.0.1:63858] script '/var/www/html/chapter3/namespaces.php' not found or unable to stat
[Wed Oct 06 01:35:05.608696 2021] [php7:error] [pid 31] [client 172.21.0.1:63856] script '/var/www/html/chapter4/namespaces.php' not found or unable to stat
[Wed Oct 06 01:35:15.729637 2021] [php7:error] [pid 19] [client 172.21.0.1:63862] script '/var/www/html/chapter4/namespaces.php' not found or unable to stat
[Wed Oct 06 03:06:49.384397 2021] [php7:error] [pid 18] [client 172.21.0.1:64066] script '/var/www/html/chapter4/autoloader.php' not found or unable to stat