Handling of ~/.XCompose broken in 21.04

When starting gvim on (X)Ubuntu 21.04 I get a long list of GTK3 error messages. It starts with

    include in Compose files not supported: include "/usr/share/X11/locale/en_US.UTF-8/Compose"

and this is followed by a long list of messages like this:

    Need to double-quote the value: "ಠ_ಠ" # LOOK OF DISAPPROVAL: <Multi_key> <o> <underscore> <o> : "ಠ_ಠ" # LOOK OF DISAPPROVAL

... for every compose combo in the .XCompose file. I have attached the offending .XCompose file.

This used to work in Ubuntu 20.10 but not anymore in 21.04.

I realize this is probably more a GTK3 error than a vim error, but this is where I encountered it, and I could not find an obvious GTK3 package to report this under.

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: vim-gtk3 2:8.2.2434-1ubuntu1
ProcVersionSignature: Ubuntu 5.11.0-18.19-generic 5.11.17
Uname: Linux 5.11.0-18-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu65.1
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: XFCE
Date: Wed Jun 9 13:41:16 2021
InstallationDate: Installed on 2017-02-04 (1586 days ago)
InstallationMedia: Xubuntu 16.10 "Yakkety Yak" - Release amd64 (20161012.2)
SourcePackage: vim
UpgradeStatus: Upgraded to hirsute on 2021-06-08 (0 days ago)
modified.conffile..etc.default.apport:
 # set this to 0 to disable apport, or to 1 to enable it
 # you can temporarily override this with
 # sudo service apport start force_start=1
 enabled=0
mtime.conffile..etc.default.apport: 2017-06-24T17:37:24.020007