Spontaneous didClose events

I’m using a commercial language server which takes a while to start up, so I typically leave it running in a tmux session for days.

After updating last week I noticed that after a while the LSP stopped showing diagnostics and hover info.
Running a :e did fix it.

I looked into the LSP logs and see every now and then a textDocument/didClose notification being sent to the LSP. I’m not sure what is triggering it, but it seems to happen after returning to neovim after a period of inactivity.

Any ideas on how to debug this? Could one of my plugins trigger this? I’m using tmux-plugins/vim-tmux-focus-events to trigger BufEnter events.
Or could this be a bug in neovim?

It happens every time the file is touched outside of neovim (like after doing a git pull/checkout etc).