Yes, I think that’s quite a likely scenario - if you were running out of disk space, Windows deleted some files from the _RESTORE folder. It could be caused indirectly by avast! - if it was unpacking some archives to scan their content, it used some space for the temporary files.

I think there is a little misunderstanding here - the Log Viewer displays various important avast! “runtime events” - such as errors occurred. It’s meant for troubleshooting - checking what went wrong.
However, it doesn’t display the “results” of the scanning - that’s what the report files is for. The report files (either TXT or XML files, according to the settings) are not “viewed” using the Log Viewer.