Now having repeated the clean install process, I am not so sure the installer didn't keep the activation code. Like before, I copied the activation code and pasted it into the appropriate spot during the install, and later after it was installed and did the initial Smart Scan, I went to settings -> subscription, and I saw that it was showing the "Have you already purchased an Avast product?" but I didn't notice whether or not the Avast Internet Security was already active, so I supplied the activation code a second time.
This time this was with beta version 17.4.2294 (build 17.4.3482.0), using the online installer for Avast Internet Security that I downloaded about 90 minutes ago.
UI suggestion: list the registered products above the question, "Have you already purchased an Avast product?", or put some notation on the same line as the product.