@dwh,
All the online installer does is download a small item which then downloads and installs the offline installer.
That suggests they may have put their "tweak" in the "small item." I think it's more work than I want, because going back requires removing Avast, which in turn involves reconfiguring everything, but I could do that using the off-line installer, to compare both update methods, to see if there's any difference in the reported Avast build number. (Right now "About Avast" is reporting "Program version" is "19.7.2388 (build 19.7.4674.524)" which should be the on-line installer result.)
Nonetheless, there is clearly a different result depending, whatever the cause. There should not be IMO.