Aluminium cans are lined with a thin polymer coating so that the beverage never touches metal. That coating is where cannabinoid goes missing.
Cannabinoids are lipophilic. Given the choice between water and a polymer surface, a droplet that touches the liner tends to stay there. The effect is called scalping, and the amount lost depends on the liner chemistry, the droplet size of the emulsion, how well the droplets are held in the water, and time. It is worse with large droplets and with emulsions that have started to cream, because more droplets reach the wall.
You can measure it. Fill a set of cans and a set of glass bottles from the same batch on the same day. Test both at fill, then at intervals over the shelf life. The glass tells you what the emulsion itself is doing. The gap between glass and can is scalping.
If the gap is large, the options are a different liner, a different emulsion, or both. An emulsion with very small, uniform, well-stabilized droplets loses far less to the wall because far fewer droplets ever reach it. Adding extra cannabinoid to compensate is the worst option: it raises cost, it makes the label inaccurate at fill, and it does not stop the loss.