Cinnamon keyboard lost ability to enter 3-rd level characters

[timon]

Distribution

LMDE 7

Package version

6.6.6, 6.6.7

Graphics hardware in use

No response

Frequency

Always

Bug description

After yesterday's update/reboot (some updates I have installed couple of weeks ago, but was too lazy to reboot then) I'm no longer able to use level 3 to enter combining characters.

When I need to type a German letter with umlaut, say, ü, I used to do Alt-U u, and that woud progress with a sequence of ¨ → ü
After the recent updates, it does not work this way: the first keystroke results in nothing; the second one produces literal u

Here's my xkbmap configuration:

$ setxkbmap -query
rules:      evdev
model:      pc105
layout:     us,ru
variant:    mac,mac
options:    terminate:ctrl_alt_bksp,altwin:swap_alt_win,grp:caps_toggle,lv3:ralt_switch_multikey

What's interesting is that according to xev, the umlaut was meant to be there (or maybe I am misinterpreting its output)

XEV dump

KeyPress event, serial 37, synthetic NO, window 0x7000001,
    root 0x4a8, subw 0x0, time 95503480, (35,-15), root:(133,111),
    state 0x10, keycode 108 (keysym 0xfe03, ISO_Level3_Shift), same_screen YES,
    XKeysymToKeycode returns keycode: 92
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyPress event, serial 37, synthetic NO, window 0x7000001,
    root 0x4a8, subw 0x0, time 95503572, (35,-15), root:(133,111),
    state 0xd0, keycode 30 (keysym 0xfe57, dead_diaeresis), same_screen YES,
    XLookupString gives 2 bytes: (c2 a8) "¨"
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: True

KeyRelease event, serial 37, synthetic NO, window 0x7000001,
    root 0x4a8, subw 0x0, time 95503632, (35,-15), root:(133,111),
    state 0xd0, keycode 108 (keysym 0xfe03, ISO_Level3_Shift), same_screen YES,
    XKeysymToKeycode returns keycode: 92
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 37, synthetic NO, window 0x7000001,
    root 0x4a8, subw 0x0, time 95503660, (35,-15), root:(133,111),
    state 0x10, keycode 30 (keysym 0x75, u), same_screen YES,
    XLookupString gives 1 bytes: (75) "u"
    XFilterEvent returns: False

KeyPress event, serial 37, synthetic NO, window 0x7000001,
    root 0x4a8, subw 0x0, time 95504132, (35,-15), root:(133,111),
    state 0x10, keycode 30 (keysym 0x75, u), same_screen YES,
    XLookupString gives 1 bytes: (75) "u"
    XmbLookupString gives 1 bytes: (75) "u"
    XFilterEvent returns: True

KeyPress event, serial 37, synthetic NO, window 0x7000001,
    root 0x4a8, subw 0x0, time 95504132, (35,-15), root:(133,111),
    state 0x10, keycode 0 (keysym 0xfc, udiaeresis), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 2 bytes: (c3 bc) "ü"
    XFilterEvent returns: False

KeyRelease event, serial 37, synthetic NO, window 0x7000001,
    root 0x4a8, subw 0x0, time 95504200, (35,-15), root:(133,111),
    state 0x10, keycode 30 (keysym 0x75, u), same_screen YES,
    XLookupString gives 1 bytes: (75) "u"
    XFilterEvent returns: False

Steps to reproduce

1. Configure English (macintosh) as keyboard layout (this gives umlauts on Alt-U)
2. Configure XKb options -> Key to choose 3rd level: "Right Alt; Shift+Right Alt as compose"
3. Try typing RAlt-U u

Expected behavior

First, umlaut/diaeresis appear without moving the cursor, then letter u below, forming ü

Additional information

No response