My old company did that. PR sent an email to all staff saying "please leave us a good review on Glassdoor". I checked and the week before four new reviews had come up, all using the same writing style, all incredibly positive. Stuff like "negatives: you have to do lots of hours, but you're paid well so I don't mind" (spoiler alert: only half of that sentence was truthful).

I reported all of them to Glassdoor and sent them the PR email, the reviews are still up months later.

edit: just checked again and everyone left fairly truthful 1* reviews in a short period of time a few weeks after I left, I'm guessing they did another round of "please leave positive reviews" and it backfired.

This exactly.

I worked for one company where everyone who left started posting bad reviews. The next thing you know people who were still there were posting wonderful reviews and getting an "official response" from the company.