Check the size of the patterns (pattern length) then. You will only get previews for notes in the matrix which are in the visible pattern length.
And Renoise will only adjust, modify, clone the pattern legnth if you’re inserting new patterns.
D’OH! That was the problem. The original blocks were 288 lines and the final destination was 192 lines, and the original blocks only contained an upbeat so there was no content at the beginning of them.
Another reason why we need matrix block height to vary with pattern length.