Help: delete-other-windows only working from the original window

Question

Overview

I'm having trouble understanding from my current config why C-x 1 (i.e., M-x delete-other-windows) does not work when executed from a window in a project that was created as a result of either C-x 2 (i.e., M-x split-window-below) or C-x 3 (i.e., M-x split-window-right).

It does work as expected when either running vanilla emacs (emacs -Q -nw) or when not having navigated to files via project.el. So there's clearly a package that's introduced a change in behaviour for this in preventing delete-other-windows from doing just that.

Is it winner-mode or treemacs or lsp or something else?

How do I trace where a no-op is occurring?

Investigations

1. I start emacs
2. Run the command find-file via C-x C-f to open a file
3. Run the command split-window-below via C-x 2
4. Run the command other-window via C-x o to switch focus to the new window
5. Run the command find-file via C-x C-f to open another file
6. Run the command delete-other-windows via C-x 1 to collapse all other windows and maximise the current one
7. This should leave the the currently focused window and the other file as the only one visible

This works.

If I replace steps 2 and 5 by running the command project-find-file via C-x p f (after having switched to a file in a project... e.g., via C-x p p f) this also works.

If I enable treemacs and open files from it, it also works.

If I run a scala spec via lsp-metals, using lsp-any-lens to select the relevant lens, it still works.

Summary

At some point (which I'm still trying to figure out) while working within a project, the behaviour changes such that delete-other-windows stops working if, as stated above, the window that has focus was created as a split.

So it's a bit baffling and I'm not sure how to reproduce it (yet).

Any pointers on being able to turn on tracing or something when it starts to fail would be great.

Even better, knowing how to force the behaviour would be even better. Thanks!