Yes it would be great for it to be resolved quickly, why it has happened after working correctly in previous program versions (free or paid) is somewhat disappointing.
Personally I feel on-demand scans in a resident AV is much depreciated. Generally I only run them if I'm beta testing (and I have stopped that) and as a result of questions in the forums.
Whilst compiling this I started a Full Virus Scan on my win10 system with avast! free 19.6.2383 installed and that is currently running (at 31% and counting) and no immediate finish as yours. Now at 50% and still going, I guess it should continue through to completion from here.
90%
What avast version are you running, 19.6.2383 being the latest ?
100% and 'Great No issues found. This scan took 9 minutes and tested see image 2.