I don't know if it is a bug. I putted 4 eicar zip files in one folder of my flash drive and scanned it in 4 different ways.
Firstly, right clicking the drive letter of my flash drive in my computer. It was good. It detected the 4 infected files. So no issue with this one.
Secondly, I used "Removable media scan" in avast user interface. Scan completed but there were some files could not be scanned. When I clicked the "show results", it was my dvdrom drive so nothing to worry about it since no cd/dvd inserted to my dvdrom drive. But it didn't found the eicar zip files. I also noticed that the amount of data tested was less than the amount when scanned it using the right clicking the drive. Going back to the dvdrom drive, better if the avast will not anymore alert the user about some files could not be scanned in case of empty dvdrom drive. It makes the user confuse.
Thirdly, I scanned it using "Select folder to scan". I checked the drive letter of my flash drive and nothing found. The amount of data tested was similar to my second method.
Lastly, I also scanned it using "Select folder to scan". I checked the folder where the 4 eicar zip files located and again, it did not found.
I did not change any settings in avast user interface.
The "removable media scan" is very helpful to people who need to scan 4 or more removable drives like me. In my opinion, the 4 methods I showed should arrived in detecting the 4 eicar zip files. So I am worried to the results I got.
I am currently using the avast! free antivirus.