Would filter-out buffers that haven’t been seen since re-opening the session, wouldn’t it?
Yes, you’re right. So this would mask your issue, but not solve it.
It seems like you’ve tried most of the logical things, and I’m not a lspconfig dev so it will be tricky for me to give a lot more help. On my system, I also have a socket at /var/run/user/$UID/nvim.XXXXX.X (where the X are random numbers). But I think that is related to nvim RPC, so probably not relevant. Because the issue seems to come from the lua LSP config, you could also check :checkhealth lsp and the LSP debug log with :=vim.lsp.set_log_level("debug") (the log file can be opened with :LspLog).
I’ve just realised as well that the recommended lua_ls snippet in lspconfig has changed in neovim v0.10, so this whole LSP system still is a bit of a moving target. Maybe if you are able to get some debug logs then they will take you more seriously in that github issue.