On our server, we made a large floor out of double carpet (ie, a layer of carpet on solid blocks, and a layer of carpet floating directly above that) in order to avoid spawning and lightning strikes.  Since updating to 1.15.1, this carpet has not prevented occasional pillager patrols from spawning, even when none of their spawning conditions are met.  They spawn at night or at day, sometimes even spawning on the LOWER carpet layer (and thus sticking out the top layer).

Pillagers are intended to spawn on snow layers, but I don't believe spawning on carpet (especially when this causes them to be halfway through another carpet layer) is intended.  It would be great to understand this issue or see it fixed.