You have read the guidelines and find nothing prohibiting shopping the location which you shopped before as a reveal, for a different MSP? If there's no mention, then you need to go by gut. If you think you can do the mystery shop without being 'marked', do it. If you have any hesitancy, don't. You could end up doing the shop, being identified, and having your shop rejected as well as being out the amount spent. Possibly, you could be deactivated from the MSP. Or, if identified, you may be prohibited from shopping the location in the future.
Maybe you should email the scheduler. Explain your concern, without naming the other MSP, and present the question.