I reported this "ages" ago (many times) with zero feedback......got finally tired of reporting with zilch feedback.
Also, another bug is Avast's UI also pops up on screen after scan....scheduled scans used to stay minimized when done if all OK.
Also, some of the times you get a false "Threat Detected" even when Scan shows fine in history.
Bottom line,
all the above seem to be faulty with Avast on scheduled scans.
So, only thing I can tell you on scheduled time....this happens with the "standard/default" scans.
I found if you create a Custom Scan (even if same settings as one of the default ones) you can schedule and it'll run on schedule.
I do a scan with Avast & MBAM Pro every morning at 4am & 5am before I wake up on all my PCs.....and they work on time.
You can see that while Avast & MBAM active shields work fine together I don't want the scanners running at the same time.
P.S. For those that have MBAM Pro and are/will do the same.....make sure in MBAM Settings of the Scan you uncheck "check for updates before scan" or it will also not run on selected time.....BUT unlike Avast, MBAM does acknowledge this is a known bug. If you are updating MBAM database throughout the day this is not big deal.