There is a bug report 1845332 about this at launchpad.
Could you please mark it also affects you?
Upd: There is a response for this bug report.
How to make Compose key combination generate strings (Ubuntu 18.04):
Settings → Region & Language → Manage installed languages → (Language tab)
Keyboard input method system to
It supposed to work also for distributions based on Ubuntu.
If XIM is working better, why it is not default?
(see answer for original bug report)
For keyboard input Ubuntu can use XIM (X Input Method) or IBus (Intelligent Input Bus) or none.
XIM is very old, it was designed for X (X Window System). If/when Ubuntu switches from X to Wayland it will be most likely dropped.
The default keyboard input method now (18.04) is IBus, because Ubuntu uses GNOME for GUI and GNOME relies on IBus to a large extent.
When IBus is selected for keyboard input method, GTK (not IBus) deals with Compose key combinations. GTK is not able to handle multi character strings. So it is not an IBus bug, it is GTK feature. No fix is expected here.
From the other side, XIM can make problems too, and most likely they will not be repaired either.
So far XIM works for me. I wonder what will happen with this issue if XIM will pass away.