You may have a lingering tmux session that has an nvi editing session locked to that file.

You can verify this with lsof <file> to determine which process still has a reference to the file.

In this case,

• tmux ls to list all windows
• tmux kill-session -a to exit all windows except current

After exiting all windows except the current one (which does not have the read-only file open), nvi <file> did not warn about read-only as desired.

Thanks to @DBen.Knoble for helping me down this trail.

You may have a lingering tmux session that has an nvi editing session locked to that file.

You can verify this with lsof <file> to determine which process still has a reference to the file.

In this case,

• tmux ls to list all windows
• tmux kill-session -a to exit all windows except current

After exiting all windows except the current one (which does not have the read-only file open), nvi <file> did not warn about read-only as desired.

Thanks to @DBen.Knoble for helping me down this trail.