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Thank you for your replication of the issue. Avast should scan the new file, but it didn't. I hope avast could shed some light on this.
You're welcome.
1. Although my assumption needs clarification - avast will scan executable files that have had the file type changed to say a .txt file type as windows (I believe ) doesn't rely on just the file type to see what should launch that file. If avast are doing this it is reasonable to believe they would do the same for .com files that are actually text files.
One thing for sure is it would be confusing for users if they start to run the eicar test.
2. As Alikhan mentions it is reasonable to delay the scan when it is written to disk (as it isn't active), but you experienced more than that it didn't scan it when executed (also confirmed on my system).
If this is the current practice then avast needs to change the avastUI > Component > File System Shield settings as this states 'Scan files when writing' (default enabled). When a file is saved to disk (download) it is written to disk, so a user would expect it to be scanned.