I have noticed slight color issues with my exports ever since the latest big update to Lightroom Classic but recently noticed a huge difference with reds, purples and magentas. The image looks entirely different in the develop module compared to the export (exported with sRGB color profile).
Here is the file with sRGB - the magenta details of her outfit are lost.
This is what it looks like in Lightroom:
Here's another screenshot with the exported image on the left and the lightroom develop module on the right. Another photographer tested the raw file in her version of lightroom and experienced the same issue as me. This session was shot with a variety of camera bodies and the issue is consistent so it isn't my specific camera.
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The magenta color simply cannot be represented in sRGB color space. You can check this by soft proofing to sRGB and turning on the gamut warning. Anything outside of sRGB cannot be represented in a sRGB file and you will likely use detail there.
Here is a video (for an older version of Lightroom) that shows how to deal with this problem: https://www.youtube.com/watch?v=-v4Z_6Y2Kbk
You can also simply use a wider export color space such as DisplayP3. Almost every browser nowadays color manages and many people's displays are wider than sRGB.
Yeah the entire dress is outside of sRGB gamut. Here is your iMac screenshot proofed with gamut warning to sRGB in Photoshop. The dress, parts of her face, and some of the flowers cannot be represented in sRGB space.
Interestingly you can only notice this because of the display on your iMac being a wide gamut display with probably displayP3 gamut. On more standard and older screens you would never have seen the difference since they are typically close to sRGB. Almost all macs and iphones/ipads nowadays have P3 gamut displays and almost all apps (even on the iOS devices) are color managed. This results in deeper possible colors on the display in the red parts of the spectrum. The situation is not quite as good on Android devices with spotty color management but nowadays even on windows machines most have wider gamut screens and the browsers are color managed so you can typically use files in displayP3 profile online nowadays and expect it to work correctly. Here is an interesting webpage that shows that you can indeed do this and that you get better color: https://webkit.org/blog-files/color-gamut/