Panel "urgent notification" flash/highlight is broken on Mint XFCE for Mint-L and Mint-Y

[CO2-Codes]

I am running Linux Mint 22 XFCE edition, which I upgraded from a fresh install of 21.3. XFCE is on version 4.18

When an application renders an "urgent notification" event, both the title bar and the item in the xfce panel are supposed to flash or change colour. This works correctly for some themes, but not for others.

How to reproduce

1. Go to Window Manager Tweaks, and under Accessibility, turn on Notify of urgency by making window's decoration blink. I'm not sure if this is strictly necessary but this is just to exclude this setting interfering with the testing.
2. Open up an Xfce terminal.
3. In the terminal, go to edit -> preferences and under Advanced make sure Visual bell is enabled.
4. In the terminal itself, run this command: sleep 2 && printf "\a" . This waits for 2 secs, then prints the bell character to trigger the notification.
5. WITHIN 2 SECONDS, click some other window so that the terminal is no longer focused.
6. Open the Appearance settings and switch between the themes to see for which it works and for which it doesn't

From my testing, everything works correctly (I see the highlight) on Adwaita, Adwaita-dark, High Contrast and also the Mint-X series.

However, no flashing/highlighting happens on the themes Mint-L series and Mint-Y series. So this bug affects Mint-L and Mint-Y.

Here you see the correct behaviour on Adwaita-dark: "Appearance" is the focused window, and Terminal is highlighted because of the bell.

Here, you see the bug on Mint-L-Dark-Grey: Appearance is still the focused window, but even though Terminal is highlighted and the title bar is flashing, there is no indication whatsoever on the panel. In fact, it looks identical to the Desktop item next to it.

Unfortunately Mint-X only comes in bright themes and I prefer a dark one. So, for now I'll have no choice to switch to Adwaita-dark.