Steps to reproduce the issue

Create 2 libraries with the manifest libraryname field as follows:

<libraryname>mycompany/mylibrary1</libraryname>

<libraryname>mycompany/mylibrary2</libraryname>

Both successfully installed and accessible / usable by my component.

Make a change to mylibrary1 and install the new version (do not uninstall ... just run the installer).

Expected result

mylibrary1 updated.
mylibrary2 not changed.

Actual result

mylibrary1 updated.
mylibrary2 files in JPATH_SITE/libraries/mycompany/mylibrary2 removed.

Have to uninstall both libraries and install both again.

System information (as much as possible)

Windows 10
PHP 7.4

Additional comments

Problem at lines 310-314 of Installer/Adapter/LibraryAdapter.php

These should read:
if (2 === \count($elementParts))
{
Folder::delete(JPATH_MANIFESTS . '/libraries/' . $elementParts[0] . '/' . $elementParts[1]);
Folder::delete(JPATH_PLATFORM . '/' . $elementParts[0] . '/' . $elementParts[1]);
}