while Avast was trying to update flash for me, I got this popping up. Has it happened for anyone else?
File reputation warnings may happen on new files … that is what reputation is about, new so we dont know it avast say 
if you trust it, allow it
you may test suspicious file(s) here www.virustotal.com / www.metascan-online.com if tested before, click rescan for a fresh result
I download flash player updates direct from source and don’t get this
By downloading from alternative source it has less reputation (usage by avast users) and the file isn’t digitally signed, this already adds to its file reputation (or lack thereof). Also the file name being downloaded differs from that of the Adobe Download. https://www.adobe.com/products/flashplayer/distribution3.html
Because this is a file reputation alert, I rather doubt it will be detected by an on-demand scan (by avast) on VirusTotal.
Some old info here (avast 7) https://blog.avast.com/2012/03/20/autosandbox-why-are-you-annoying-me/
In addition, this feature is connected to the FileRep cloud feature which identifies new files for additional analysis.
AVAST Software: Detection is faster when FileRep knows all the clean files
https://press.avast.com/avast-software-detection-is-faster-when-filerep-knows-all-the-clean-files