Missing action button icons

Xubuntu Linux 21.10 Linux 5.13.0-21-generic #21-Ubuntu SMP
Audacity 3.1.1
compiled from source code according to BUILDING.md. compiled with “RELEASE” flag.

Issue is triggered when Audacity opens a dialog box with action buttons
Expect icons next to the text in the dialog box. Instead, see only a generic broken icon. (see “Cancel” and “Open” buttons below)
also, gtk errors are seen at startup:

Sorry, I don’t know what the problem is. It works fine for me with a Release build of 3.1.1 on Xubuntu 20.04 LTS.
You could try asking on the developer’s discord channel: https://discord.gg/5MJyAbQv