Yesterday, when I placed the number 10 in the min level field and kept the max field empty, I would see a full page of mostly level 10 quests. Now today. when I do the same, I barely get any level 10 quests. I see mostly much higher level quests. It is very hard to find all of the level 10 quests because they are spread out among many, many pages of search results.
Placing 10 in the min level means you are starting at level 10, and then not placing a number in the max level means you are searching from level 10 to max level which includes a ton of results.
What you want to do is put 10 as the max level and keep min level as 0. That will show you quests that have the levels set as 10 and below.