My colors sporadically mutate when I post to web

I have been putting up with an annoying problem, and would like to wrestle it to the ground. 

I shoot all my files in RAW format, either Pentax (PEF) or Olympus (unsure of extension). I process them in Lightroom Classic on an ASUS laptop with a calibrated screen, running whatever garbage operating system Redmond is currently forcing down our throats.

I export them, using Lightroom, into JPEG format. When I want to make a print or something to hang on my wall, or my current project of making some postcards, I use Walmart or Costco or Moo, or some similar service. 

Here's the rub. On my screen, the colors are bright and clear and crisp and true. When I load them onto the website at Walmart or Moo, sometimes they look just fine, but more often than not they mutate. They desaturate and lose their contrast and brightness, and sometimes seem to even shift in hue. 

I have learned that if I order prints from these uploads that mutate, the prints are mutated just like the website preview. Yet when I open the same file on my laptop, the colors are bright and crisp and true.

Help me, Obi-Wan Adobe! You are my only hope!