Thank you for the news. This worked for me up until two months ago. Then suddenly I started seeing this warning message we are talking about on most HTTPS websites.
I don’t agree, that Avast works as it should. I am also eligible for enjoying the benefits of “transparent” HTTPS scanning, as it is advertised in the release notes, and I want it back. 
Also, in my other post I have written, that I experience the bug with all Firefox add-ons disabled too.
Here is a list of UK banks obtained from Wikipedia: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16. Alternatively you can open all of them at once if you copy-paste this into a cmd.exe window:
"C:\Program Files\Mozilla Firefox\firefox.exe" https://www.atombank.co.uk/ https://www.bankofireland.com/ https://home.barclays/ https://www.hsbc.com/ https://danskebank.co.uk/personal https://www.lloydsbankinggroup.com/ https://www.metrobankonline.co.uk/ https://monzo.com/ https://www.natwestgroup.com/ https://www.santander.co.uk/ https://www.starlingbank.com/ https://www.tandem.co.uk/ https://www.co-operativebank.co.uk/ https://www.tsb.co.uk/ https://uk.virginmoney.com/ https://www.sainsburysbank.co.uk/
For me all of them show the warning message, except Virgin Money UK (15). @rocksteady told that for him HSBC UK (4) worked. For me it doesn’t. So, the list of websites, that work is not the same for all users.
I have also tried Japanese banks (source: Wikipedia): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15.
"C:\Program Files\Mozilla Firefox\firefox.exe" https://www.dbj.jp/ https://www.jfc.go.jp/ https://www.okinawakouko.go.jp/ https://www.jp-bank.japanpost.jp/ https://direct.jp-bank.japanpost.jp/ https://www.mufg.jp/ https://www.smfg.co.jp/ https://www.smbc.co.jp/ https://www.mizuhogroup.com/ https://www.chibakogyo-bank.co.jp/ https://www.smth.jp/ https://www.resona-gr.co.jp/ https://www.aozorabank.co.jp/ https://www.boy.co.jp/ https://www.82bank.co.jp/
For me all of them show the warning message, except your example (https://direct.jp-bank.japanpost.jp/) which I included as number 5.
Actually, you don’t have to hunt for the warning message. You see it naturally, if you are used to verifying the company name in the Site information popup, every time you connect to a website where you enter sensitive data. Now the warning message is there instead of the company name you want to verify.
From the release notes @NON quoted, we know, how it should work. It doesn’t work that way for some users (e.g., you and me), therefore I agree that this must be a bug. I suggest reporting it in Avast (Menu > About > Got any feedback? LET US KNOW button), referring to this forum thread, as we collected a lot of information useful in investigation, with screenshots.
I will not open new threads on this topic.