Subsetting a reference frame gives a plain data frame.
Source:R/calibrate.R
sub-.gr_reference_frame.Rd[ on an object whose class extends data.frame keeps the CLASS and drops
every other attribute, so ref[, cols] came back still claiming to be a
gr_reference_frame while of, frame_n, screened_n and seed were gone,
and gr_calibrate() then computed the corpus-wide metric set from a stratified
sample. The same trap, and the same fix, as [.gr_gaps.