systray: Some(?) icons no longer scaled for HiDPI

I have a HiDPI setup with 2x scaling and since the update to 4.18 the systray icon for Slack Desktop (didn't notice this with any other application) is no longer scaled correctly. This used to work.

Screenshot

Details

Arch Linux x86_64

Package versions:
libxfce4ui 4.18.0-1
libxfce4util 4.18.0-1
xfce4-panel 4.18.0-1
gtk3 1:3.24.35-2
slack-desktop 4.29.149-1

Faulty update:
[2022-12-19T17:17:08+0100] [ALPM] upgraded xfce4-panel (4.16.5-1 -> 4.18.0-1)