I'm not sure when they do the updates. I can confirm that when I used it on 4th November 2022 it functioned correctly, the following week it didn't so something happened over that weekend.

I'm mainly a Mac user on Safari so when you click on the attachment it briefly opens a new tab, saves the attachment to the Downloads folder and closes the tab again. I then tried it on Chrome on the Mac and basically the same but it just downloads it. Then changed platforms to Windows 10 on a PC again using Chrome and then Internet Explorer with the same results. 

Companies House continues to open PDF's in new tabs as does other online software so rules out browser setting problems.