Here is some troubleshooting ideas I have. Do any of them make any difference whatsoever
Using a very generic, well supported icon theme with no customization. Is the problem that tint2 isn’t picking up any icon theme or it just isn’t picking up these icons? If not picking up any icons, then what icons is it using?
create a fresh user on system with all default settings and see if anything is different? And if you have done system level customizations you could even try a fresh system liveboot/VM but that is a lot of work to be fair.
change your xfce4-appearance and xfwm-settings themes
Do you install icon themes for the user or the system? Try the other way does it make a difference?
I am using latest stable xfce4 (1.18 I think) with tint2 for task list and xfce4-panel for workspace switcher miniature view. From what I’ve noticed, the icons in both are always the same between the two of them. Tbh in general I have found changing icon themes very annoying and inconsistent across the system with some applications and tray items being resistant. Like firefox developer I find is impervious to icon themes. Do you have the issue with all applications including very well-supported ones like thunar or mousepad?
There is a command called gtk-update-icon-cache/gtk4-update-icon-cache but I don’t know if it would help.