Bug or misconfiguration?
That's kind of up to you.
If Avast is scanning more things than it once did, it would be hard to call that a problem.
You have invited the software into your computer to protect you. The software is fully configurable to, for example, exclude certain kinds of checks of certain files / folder trees - it all depends on you to set that up. You can, for example, choose to be slightly less well-protected in order to gain back performance simply by excluding certain folder structures from certain kinds of File Shield operations.
Like Eddy, I have no idea what SUMo is. My note above, however, applies to pretty much anything you do. I recently chose to exclude certain files/folders I use heavily during software development, as I'm not developing virus software and I don't need my own build products, etc. scanned repeatedly while I'm waiting for a build to complete. Thus, I chose performance over the (small) possibility that a file I'm accessing during such operations could become infected. But I did keep Execute checking turned on, and of course I kept Avast's Web Shield turned on.
-Noel