I’ve just noticed that if the exposure time is sufficiently fast, then the metadata for the exported TIFF file lists the exposure time as 0.0 seconds (I think the value is not being recorded).
The cut-off happens somewhere between 1/60 and 1/150 (i.e. TIFFs with an exposure time of longer than 1/60 carry the correct metadata).
It happens with both DNG and RW2 raw files.