Avast doesn't scan Eicar.com.txt automatically

i even tried it with scan all files open, all files write, option, still no succeed.
only scan from right click works, but may be you need add scan files with double extensions.

You may want to check the list of exclusions of the File System Shield - *.txt is there by default.
Double extensions are dangerous only if the last extension is - there’s no point in scanning File.xxx.txt, no matter what xxx is.