That makes perfect sense and coincides with all the trouble when Hulu was getting phased out for older Roku models. They offered a paid upgrade for something that was incompatible with the old system but Roku customer service was no help figuring out a workaround.
It is weird that this issue manifests in this odd way.
It appears that Roku is deliberately sabotaging older units to force 'legacy' users to upgrade. Multiple long calls to Amazon indicate they are clueless and consider this a Roku problem, so that means Amazon Video really doesn't care about a few Roku users who haven't upgraded their equipment. This all started immediately after Roku offered their 'upgrade promotion' a few months ago; that forced upgrade was really an accommodation of Netflix' changes to their stream/interface and it looks like Amazon Video was crippled as collateral damage.