I might have edited too much of the original post so I post that message to answer my own question, trying to gain clarity and hoping to make it clearer for other users stomping upon that very same problem.

I did have both ruby versions, the first one being 3.3.8 installed via the xbps-install VoidLinux's package-manager and the second one being 3.5.0dev, built from source.

The solution was to nuke the previous ruby installation and all its environment, including the following packages: ruby, ruby-lsp, ruby-manpages, ruby-multi_xml` and ruby-ri.
I also ditched both /usr/lib/ruby` and /usr/lib64/ruby dossiers.

I then removed all environment variables related to Ruby (viz. RUBY_ROOT, GEM_PATH, GEM_ROOT, RUBY_GC_LIBRARY, GEM_HOME and RUBY_CONFIGURE_OTS).

I then carried out gem update --system then gem update, and finally gem install ruby-lsp.
I removed the bin's path of solargraph (another ruby LSP) from my $env.PATH as only directories - not binaries - are used there.

Thank you for your time.