Canon R7 with 800mm and 2x Extender - stabilization issue

I realize this is a kind of extreme setup (2560 handheld).  Normally, I'll be using a tripod. But this week, I went out to test it for handheld shots.  And discovered something I'm wondering if it's a fault.

First, I have used this lens before (not this specific one, but an identical model), with extender. Never had any issues. I've used it with the R5 and now i'm testing it with the R7.  I no  longer have access to the R5 to test that again. 

The Problem: At 800mm, with 2X extender, it's all I can do, to keep an image in the viewfinder.  I've turned stabilization on and off, and I can see it trying to stabilize the image.

But the real issue is that it doesn't seem to be a problem, all the time.  And I can't figure out what the reason is.  Last weekend I took it out to take photos of an eagle, about 150 yards away.  It worked great.  Not a single issue with image stabilization and focus.

Yesterday, I took it back to the same spot, under almost identical lighting conditions. It was a snowy background and lots of light. But this time, the lens struggled to focus, and hold the eagle in the frame.  To be fair, it was a tad bit foggy. Seemed clear enough to me but to the camera there may have been too much fog, albeit, seriously very little.

So, I chalked it up to conditions, and the f22 f stop.

However, today, I did another test.  In bright sunlight, on a very clear day.  It focused fine, but still struggled to hold the image stable. I tried smaller focal length lenses and they worked fine. I tried it with and without the extender, and it wasn't much improvement.  Like I said, I've used this lens (not this one, but an identical one) and never had a single instance of not being able to hold the image stable.

An extreme example: I used this setup before, on my R5, to take photos of Seattle, from the other side of the sound.  The distance is over 5 miles. I had no problem handholding the camera, and being able to see people in the Space Needle.

This weekend, I couldn't even keep that eagle in the viewfinder, from 150 yards away.

So, what I'm wondering is if this might be a defective lens, or R7 stabilization issue.  I've done a factory reset, and can't seem to, now, get it to hold a stable image.  And, like I also said, if I turn the stabilization on nd off, I can see a difference, but nothing stable as I used to be able to get.

Is there a setting that can cause this?
Are there lighting conditions that could cause this?
Is it likely a defective stabilization unit in the camera? Lens?
Maybe I could test in body vs. in-lens stabilization?  And see if one is defective.
How would I turn the camera in-body stabilization off?

Any suggestions?

Mike

Wow! No answers.

We have some sun today. I guess I'll try to do some more testing, with and without the extender.  I have used this lens before, many times, with the R5 (no longer have access) and never had an issue. Since I've bought the R7, and this lens, I've only had it work great on one day. The rest it is slow focus and very poor stabilization.

I'm just trying to determine if this is a lighting issue, a camera issue, or a lens issue.

The lens worked fine today. I'm going to chalk this up to insufficient light.

This was one of the pics today.
Distance: About 5 miles across the sound from Alki to Seattle.
No cropping. Canon R7, 800mm f22 (with 2x extender)