LrC stopped building Smart Previews on Imports

Standard previews and Smart previews are two entirely different things. Lightroom always needs to have some kind of normal preview, because that is what you look at in the grid and in any other module. Smart previews are not previews at all. That name is misleading. Smart previews are proxies for your original images, so you can edit images even when the originals are offline. That means Lightroom cannot build standard previews instead of smart previews. It can build standard previews and smart previews, or only standard previews.

Check that you've checked the 'Build smart previews' box in the Import module (see atatched screenshot for location). If the box is checked, then your LrC preferences may be corrupt. You can reset these by following the steps set out in Method 1 at https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html