Prime towers reserved depth from the prime volume alone, ignoring the flush
matrix: rib-wall towers in both the engine and the preview, and rectangle and
cone towers in the preview, which never carried the flush-aware estimate the
engine already used. The preview also read the print preset, which does not
carry the printer- and filament-scope keys the estimate needs and so silently
fell back to defaults. On multi-nozzle printers the flush matrix, which holds
one block per nozzle, was additionally read as a single block. The tower could
come out too small for the purge it has to hold.
The flush-based estimate also skipped the height-based minimum depth that the
prime-volume one applies, so low-flush prints could estimate a tower shallower
than the one that actually gets built.