The compose key approach is an OS function external to LO. You should be cautious with it. Under Linux and other *Nix (MacOS is originally based on OpenBSD if I remember right) there are two keyboard managers, one for text interface (the very basic access to your computer, reminiscent of the time when there was no graphics), one for GUI and they are independent from each other. The GUI manager takes over as soon as the graphical system is initialised.
The compose key is rather part of the TUI and uses specific tables. Under GUI (X-Window system), another technique is used, xkbd, with completely different tables and algorithms. Transition to Wayland results in different tables. MacOS may use an alternate technique as its GUI is Aqua.
I suggest you check with the LO-internal input method where you type “U+2014” (without quotes) followed by Alt+x. Under MacOS, the Alt modifier may be another key.
The strange phenomenon is the delayed transformation.