I don't think you can create a search where the last log is a DNF. I should know, since I have designed it, but sometimes complex queries can be done that I don't realize myself at first.
The filters are created with the opposite in mind here, ie, you do not want to visit caches with issues. I am not saying your idea is wrong, but this was how I thought when I did build it.
For logs there are 4 different filters, just as you mention. I'll try to describe them to make it clearer (also for anyone else who finds this question).
Number of logs - Filters on the amount of found (or equivalent) logs the cache has. Can be less than, equal to or more than an exact amount.
Max X DNF the last Y logs - Find those that has MAX 1-7 DNFs during the last 1-7 logs. Chosing 1 and 1 here would give you both those that has 0 and 1 DNFs the last 1 log. Ie, it would give you all of them. Now when I look at it, it would be reasonable that the first selectbox actually had a zero in it as well.
At least X finds in a row - This is from the latest 7 logs only, which it does not really say. So chosing 3 here would match for example DNF, DNF, Found, Found, Found, DNF, DNF.
Found last X days - I hope this is clear, the cache should have a found log the last for example 30 days.
Adding a filter that requires the last log to be a DNF would be doable. But it would take some time since the Custom filter system is quite complex, and I am not sure that I can squeeze that in right now.
I don't know what your goal is, but my best guess is that you are trying to find good targets for some challenge?
In case it's about finding caches that you own which has DNFs I would recommend Needs maintenance? instead. One of the things Needs maintenance looks for is if the last logs are DNF. But it only does so per cache owner, and not for a whole area. But I am quite sure that you know what you are doing and looking for, and that this doesn't really help you.