Eco mode turns off (but light stays on)

That is all circumstantial evidence.
It does not PROVE that ECO being on or off is making the difference he thinks he notices.

I have never been able to tell the difference except with the A/C when it's really hot.

Something funny is going on but it may not be the ECO mode itself.

 

This is the big question.
What evidence are you siting to suggest for sure that eco mode is turning off?

Also...

I wouldn't call that a "sharp decline".
That's a pretty small dip in average, that could be explained by numerous other much more mundane and common factors. Before I would be assuming the light is on....but nobody is at home....I'd be thinking about winter blend fuels, and/or something as simple as tire pressure.

But really? If you are concerned about your fuel economy, and or think the Eco mode is malfunctioning that's a "service" issue.
But before I would tell a service adviser I "think" the eco mode light is staying on, but the car is NOT in eco mode, I'd want a stronger case than a 3 mpg drop in MPG's as evidence.

800 miles a week? At all mostly highway speeds? That's a lot of driving.

My other now fading understanding, is that Eco Mode, at a constant highway speed, really doesn't have too much of an impact. Once the vehicle reaches a constant speed and maintains, many of the adjusted factors Eco Mode creates, simply don't come into play. Eco mode is mostly just a remapping of the accelerator pedal sensitivity, along with some adjustment of how the climate control interacts. There may be other factors being adjusted, but those are the biggies. And at 800 miles a week, mostly all at highway speed? I don't think eco mode, working or not working would make a huge difference.
When I owned my regular Gen 3 Prius, I didn't notice a big difference in MPG on long highway drives regardless of what mode I was in, which was nearly always "normal" or "eco". Eco seemed to help the most, in city driving, and maybe in the summer when it helped keep the air conditioning usage a little more in check.

My recommendation is, if you're concerned about your gas mileage, check the common things first. If you are still concerned? Have a service department check out your eco mode.

My suspicion is that whatever may be happening, could be a whole lot more common.