Since T161413, it is possible to create farms differentiated by paths. But 1/ two of the cached file use the host as identifier (file name), which create a situation where multiple wikis have the same Composer extensions set (file /cache/composer) and LocalSettings (file /cache/LocalSettings), which is a bug;, and 2/but the cached file in `/cache/wikis` uses as identifier the host and path in reading and host in writing, which create a situation where this identifier is never found and the cached file never used.
When hosts were the unique difference between wikis, it was possible to use the host as an identifier, but it is very different now with the per-path feature: only the beginning of the path should be taken into account to create the difference, and this prefix has a variable length.