I am on the fence about this issue because deep down (actually not deep at all) my reaction is, "Why did this have to happen on a mystery shop? How come it never happens when I am on my own dime.?" I think this feeling comes from the fact that this is a rare occurrence and if it happened on a mystery shop it will be a long time until it happens again for me and I would appreciate a free meal once in a while. Terrible food would not be worth it especially if I re shopped and had to eat a terrible meal again. On a fine dining shop after collecting all the timings and paying close attention and remembering all the quotes needed I have already done a good bit for that shop and it would not be worth it to me for a do over just a few days later. But yes, I feel the report should be done as it is important information for the client when this happens.
On the other hand there have been two times in my life as a mystery shopper when I have been comped or reported a sticky situation when I have been told I can no longer do that shop as the manager knows who I am. In one instance I was not allowed to shop other locations either.
So bottom line, I do think it would be nice (hmm maybe I need a stronger word here) if the client/msc offered some sort of compensation for your time when this happens. And a small fee with reimbursement still would not count in my mind on a report and shop that took me hours and the fee was $10. Their idea often is they are compensating your time with a great meal that they often think is a free meal for you. That free meal mentality is the part that I think keeps them from considering partial compensation. You did not get a free great meal and you did not even get the great meal they promised in return for your time and effort.