Lightroom export issue - image lighter and less contrast on export

One thing to realize is that you simply CANNOT control what your clients see. Their displays are simply all over the map. You can however control what the average client user sees. The absolute best you can do is to calibrate your display and NOT trust anything you see in unmanaged applications and only export your images to sRGB. Then only judge your images in managed applications on your monitor and NEVER trust anything you see in an unmanaged app. Unmanaged apps will only be correct if your monitor is exactly sRGB gamut and you have calibrated to the sRGB gamma curve (not typically done). However when calibrated, managed apps will be correct regardless of your monitor's gamut and gamma curve. When you do this, the average viewer will see something that is the same as what you see in a managed application. 

The counterintuitive thing for many people is that the average user's display will NOT be like your display in an unmanaged app. It is natural to think that but that is not the case. The average client's display will be closest (taken over 100's of users) to the image displayed on your calibrated display in a managed application. 

Lastly, a factor that many people forget about (and that might be at play here) is that sometimes perceived brightness and sharpness are influenced by the scaling algorithm that viewing apps use. There is unfortunately no standard for this and especially with images with lots of noise (e.g. night sky images or high ISO images), you can be fooled by this. This has nothing to do with Lightroom (which uses a quite accurate scaling algorithm) but is simply a consequence of math and how different software developers approach this. The only way to prevent these percceived brighness and sharpness changes is to export to exactly the size in pixels that your image will be viewed at so as to avoid any scaling of the image on the client side. This can be basically impossible to accomplish however.

The three new screenshots look completely identical to me.

If you had done a comparison at 1:1, you might have seen a difference in Photos, but the problem is that Photos won't allow you to view the image at 1:1. (or at any particular magnification)

I suggest that you stop using it, and stick to a color managed viewer.

so here is the issue, the client will not have a colour managed viewer on their device, the image looks like crap on their device, im not going to get paid.

You have no control over how images will display on other people's computers.

But you can tell your client that if they want to see a correct rendering of the image, they have to use a calibrated monitor with color managed software.

Another observation, the border/background around the images. Image on left gray border/background, image on right black border/background. Edit your image before export , or re-edit the exported image with the same border/background as will be displayed in the published location.

I am talking about the color behind the image,

https://havecamerawilltravel.com/lightroom/lightroom-classic-change-color-behind-image/

Ahh, this link exoplanes how and why a lot better:

https://youtu.be/tdTxdiC2078

The image is in b&w, but color management still applies.

No native applications on Windows 10 are color managed, including Photos.

Color managed applications, like Lightroom, use the monitor profile to display images correctly. They do this by converting the image from the document profile to the monitor profile.

Applications without color management do not do this conversion, and will consequently display wrong colors.

There are several third party color managed image viewers for Windows. I recommend FastStone, which is free for personal use.

Make sure that color management is enabled under Settings. (both boxes must be checked)

https://www.faststone.org/FSViewerDetail.htm

As for web browsers, all the major browsers are now color managed, except Internet Explorer and Edge.

Phones are not color managed, and can not be expected to display correct colors.

I am making the unsafe assumption that your monitor has been calibrated.

To get the exported images to look like they do in LR, the photo viewer program needs to be color managed. Unfortunately, the default photo app in Windows is not color managed. The Chrome and FireFox web browsers are color managed. Most smart phones are not color managed and tend to have their screen settings by default to be too bright and over saturated.

All that said, since this is a b&w image, color management would seem to be not the right place to look...... Hmmm.....