I have bought lots and lots of reviews for my Fiverr gig. I never had any problems ever.
Your Fiverr account has not got flagged due to buying reviews.
Fiverr has set it's system in a way that people keep buying reviews for example in order to retain a Seller Level, etc, because with each buying and each selling, Fiverr makes money, so they have set their system in a way that they make more money by setting it to make sellers to make more sales, no matter even through purchased reviews.
Fiverr system flags accounts only when a seller has made an indication to a buyer to contact directly outside of the Fiverr platform or if a seller's gig was found as violating Fiverr policies or if buyers have complained to Fiverr something about the seller or seller's gig, which violated Fiverr policies.