When accessing the admin/reports/status page, the PHP version displayed is not the one selected in your Cloud UI, e.g. you can see PHP 7.1 on this page:

... whereas the version PHP 7.3 is selected on your Cloud UI as you can see on admin/reports/status/php

This might be caused by some contrib module(s) requiring a specific version of PHP, e.g. the Trailing Slash module (at least until the version 8.x-1.0) which contains in its .module file the following

define('TRAILING_SLASH_MINIMUM_PHP', '7.1');