As one who manufactures paint, I can tell you that several forces come into play that can interupt production. The raw materials that are used to formulate the paint or the packaging to bottle the paint.
I have bottles which I cannot source caps. You'd think there is a standard, well yes there is but companies come and go as do their products. Companies try to offer a product that appeals to as many customers as they can, hence they change designs which impacts everyone using those items. A slight change in shape or size can trigger problems for the user, namely labels don't fit and have to be redesigned and sourced. Unless a company produces their own bottles, they'll run into situations like this occasionally.
The biggest barrier producers face is countries who change their labeling laws. In some cases companies get a lead time before the changes go into effect to clear the supplies in the supply chain. Sometimes the rules change immediately and those in the chain have to be pulled and relabeled. Either way it usually interupts the supply to the consumers.
I don't know if Tamiya is or isn't changing vendors, only time will tell us for sure. As popular as their paints are they must be losing money with this hiccup in the supply chain. Maybe they sold the paint division to someone else?