Where reviews actually go.
Five causes explain nearly every disappearing review, in roughly this order.
Google's spam filter (the big one)
Automated detection removes reviews that look inauthentic: posted on your Wi-Fi, from new or thin accounts, in bursts after a promotion, or via a review kiosk at the counter. It's aggressive, and legitimate reviews get caught.
The reviewer removed it, or Google removed them
People do delete their own reviews, and when Google shuts down a spammy account, every review that account ever left vanishes with it, including the real one they left you.
A content policy flag
Reviews containing links, phone numbers, profanity, or anything reading as off-topic or promotional can be pulled, sometimes weeks after they were posted, which makes the removal feel random.
Your profile changed status
If your profile got suspended, merged with a duplicate, or re-verified after an edit, reviews can hide or vanish in the shuffle. If ALL your reviews disappeared at once, look here first. Start with the suspension guide.