Ok now I get it.
I tested this and I cannot reproduce. I set it to Blue, rebooted and the blue accent is still on.
I am not surprised, as Dr460nized theme does not override your custom theme. It is applied once, during installation from ISO or when you install the package garuda-dr460nized and apply the Global Theme.
Overwriting your custom theme would not be respectful to our thinking, at Garuda we encourage people in customizing their own theme, as showcased here, for example.
There is probably something going on on your installation that resets the accent colors on reboot OR you hit a Plasma bug.
I searched and came up with this list.
I could not find anything exactly to your reboot issue, but one could be somewhat related:
As I mentioned previously, you should try to apply a different Color Scheme, reboot, see if the Color Scheme is still applied and then change the Accent Color, reboot and see if it’s still applied.
This would confirm whether it happens across all Color Schemes and not just Sweet.