I have added a workaround to the 2.9.9 version of the mage2pro/core package:
https://github.com/mage2pro/core/commit/cd3fc519
To fix the issue just upgrade your extensions.
The defect method is \Magento\Framework\Module\Dir\Reader::getFiles():
By the way, in my 2 years Magento 2 experience, the filesystem handling is one of the less quality (one of the most buggy) Magento 2 parts.
Particularly, Magento 2 uses this method to find out all the composer.json files of all the installed modules.
In this case, the $filename input argument has the composer.json value.
If a Composer package contains multiple Magento 2 modules in subfolders (as mage2pro/core), then the following code line works for such modules incorrectly and returns an empty string:
The next code line ignores the incorrect result of the previous line and concatenates in with the /composer.json string:
So the $file is /composer.json.
The following lines is an ugly attempt to detect the existence of the $file:
It is incorrect, because /composer.json is really interpreted here as the root Magento 2 composer.json file, and the detection result is true, but actually it should be false.
Locate the following code block (it is explained above):
Replace it with the folllowing one:
if ($moduleSubDir) {
$file = $moduleSubDir . '/' . $filename;
$directoryRead = $this->readFactory->create($moduleSubDir);
$path = $directoryRead->getRelativePath($file);
if ($directoryRead->isExist($path)) {
$result[] = $file;
}
}