I tried rendring 1:1 previews, but it had absolutely no effect on the issue. The graphic card drivers are the latest, and the general behavior of the problem makes it very unlikely to be linked to anything in the graphic card.
Meanwhile I noticed an additional detail: my previous description of the problem applied to situation when single-photo preview in Library screen is set to "FIT" mode. If I switch to "FILL" mode, the symptoms begin to work "in reverse". Now landscape photos are previeved shraply, but portrait photos are previewed blurrily if the previous photo had landscape orientation.