Here's a conundrum for you

Bear in mind that it is not Alan's image, it was downloaded from the OP of "edit my image" #17 and that image has no embedded profile. Therefore, the darkening has to do with Alan's use of LRC and particularly what input and output profiles were selected.

You do not choose input profile in LRC. It always use Adobe RGB in library mode and ProPhoto in Develop module. You can choose color space during export operation.

OK thanks; my computer is Adobe-free and I didn't know that but it should help Alan to solve his issue.

Don't use LR, but I would assume you set it to a preferred Profile on install. (Settings , Colour, etc, etc ??)
If this has been done, does it match the Profile you set your Windows at ?
(All my software uses a common ICC profile stored near the top of my Photos folder in it's own little directory.)
As a wild guess, but; that much difference looks like an output preset of some sort (optimise for web?).

I just looked at your third attempt in #17 - the one with the sky replacement - it has a standard matrix sRGB embedded profile type 'mntr' and looks OK on my system.

Oh ... Monty Python strikes again ... 😉

I'm glad the effect doesn't seem to be coming from my original image ;-)

If you apply EV to the image in LRC, you will see it's affect on the original image and this affect will be carried over to the exported image. I exported thousands of pictures from LRC to JPEG and never had image which was darker or lighter than original edited image.

I downloaded this photo and looked at it closely.
Color profile seems to be fine, sRGB IEC61966-2.1
It looks pretty dark on my screen too.
Then I increased the brightness of my screen and it looked fine.
The brightness of the screen is probably the reason, especially if it automatically adjusts to the lighting conditions in the environment.

Then why images in the OP screenshot on the same screen at the same time look different?

just a FYI,...
I posted 3 original images in the link (an unedited bracketed set)
The first of those 3 images was displayed in my OP. It had a -0.3 EC made in the camera, the image posted was unedited.
While taking the 3 images, the camera changed the exposure compensation automatically to have these 3 camera settings of EC -0.3 , -2.3, +1.7

I've used Lightroom since v2 in 2008, and currently have a catalog of 150K, so I've seen a lot of LR images in and outside the LR interface. Most of the time, if an image looks right on your monitor in LR but is too dark when posted online, it is usually either because of your monitor setting (which is likely NOT the case here because you'd have noticed it in lots of images), the environment you posted it to (DPRev is known to produce somewhat darkened thumbnails), or, the most likely scenario - export settings. I am subject to forgetting to check the export settings, and if I accidentally changed something, or my last export before this one was something I don't usually use, I can get a surprising result. The settings don't revert to a default, but remain at the "last used". There are of course other quirks that I can accidentally create, and Adobe annoyingly tends to release new versions before they do adequate testing to find bugs, so the first use after an update can yield bad surprises. If the problem persists, you could take a screenshot of your export settings screen and post it along with the dark image it yields. I'm sure some of us can help you figure it out.