Thanks Ryuu for your reply!
For sure that it is not a bug, everything is working as it supposed to work. As you said it's kind of workaround which I would use to save some time for work cutting rendering time with minimum loss of picture quality. Everything said above is not concerning some certain scene(s), it's pretty common thing for most of my interior scenes. Maybe it would be more clear if I illustrate what I do manually and it will make more clear what I mean saying "Render region excluded from adaptivity"
Here is very simple example scene and it's clearly illustrate the idea.
First lets check how much hairy carpet affects the render time. All pictures with adaptivity ON rendered with 4% noise level limit.
Picture 1 - no fur on the carpet, adaptivity ON - render time 2.33.
Picture 2 - carpet with fur, adaptivity ON - render time 25.35.
Picture 3 - carpet with fur, adaptivity OFF - render time 10.19. I rendered this full frame until the moment when the quality of carpet area reached acceptable level comparable with carpet on Picture 2.
Next pictures illustrate what I normally do to cut the time for rendering preliminary pictures for clients.
Picture 4 - carpet with fur, adaptivity ON, but problematic region with carpet excluded from render (this is the moment when I dream about polygonal not rectangular render region frame) - render time 3.47
Picture 5 - carpet with fur, adaptivity OFF, but only problematic region (this is the moment when I dream about possibility to invert render region (requested in Feature request)) with carpet was rendered until acceptable look - render time 5.06
In this scene I could use Render selected - Include/exclude list functionality to mark the area for render, but it works not for all cases in different scenes, so I've chosen Render Region for this illustration.
If I would render a real project I would need to blend Pictures 4 and 5 in PS (this is the moment when I dream about Render region mask element(requested in Feature request)) because of Bloom And Glare which works only with rendered region. In this particular case I've got
Picture 5 with actually the same quality as
Picture 2 in 3.47+5.06 min = 8.43 min instead of 25.35 min of render full frame with adaptivity ON.
Of course, this is very simple scene and the difference between variants of render in normal working scenes is not that huge, but anyway it saves me pretty much time if we are talking about sets of 5-8 2400x1800 pictures for clients. If this could be done automatically inside Corona during the single render session (the same picture will be rendered partly with adaptivity ON and partly with adaptivity OFF), it will save even more time not only during the work on pictures for clients, but during the working process with IR or production render of drafts.