h2o.relevel_by_frequency produces invalid results

[hutch3232]

H2O version, Operating System and Environment
Tested on latest h2o 3.42.0.3 on both Windows and Linux versions of R.

Actual behavior
h2o appears to be resetting the values of the column, not the underlying index, which results in values essentially being recoded incorrectly.

Expected behavior
For a given record, its value should not change when releveling, only the underlying index should change.

Steps to reproduce

hair_dt <- as.data.table(HairEyeColor)
# expand back out the frequency table such that N records for each combination
hair_dt <- hair_dt[rep(seq(.N), N),][, N := NULL]
hair <- as.h2o(hair_dt)
hair <- h2o.asfactor(hair)

hair
#    Hair   Eye  Sex
# 1 Black Brown Male
# 2 Black Brown Male
# 3 Black Brown Male
# 4 Black Brown Male
# 5 Black Brown Male
# 6 Black Brown Male
#
# [592 rows x 3 columns]

h2o.levels(hair$Hair)
# [1] "Black" "Blond" "Brown" "Red" 

h2o.group_by(data = hair, by = "Hair", nrow(1))
#    Hair nrow
# 1 Black  108
# 2 Blond  127
# 3 Brown  286
# 4   Red   71
#
# [4 rows x 2 columns]

hair$Hair_relevel <- h2o.relevel_by_frequency(hair$Hair)
h2o.levels(hair$Hair_relevel)
# [1] "Brown" "Blond" "Black" "Red"

h2o.group_by(data = hair, by = "Hair_relevel", nrow(1))
#   Hair_relevel nrow
# 1        Brown  108
# 2        Blond   71
# 3        Black  286
# 4          Red  127
#
# [4 rows x 2 columns]

hair
#    Hair   Eye Sex Hair_relevel
# 1 Black Brown Male        Brown
# 2 Black Brown Male        Brown
# 3 Black Brown Male        Brown
# 4 Black Brown Male        Brown
# 5 Black Brown Male        Brown
# 6 Black Brown Male        Brown
# 
# [592 rows x 4 columns]

We can see the labels moved, but the underlying indices did not, so nrow for index 1 is 108 for both (Black in the original, and mistaken Brown in the releveled example). Looking at the transformed hair data, we can see Hair_relevel has a different value which is certainly wrong.

Add any other context about the problem here.
A relevant issue that I believe was incorrectly closed: #6853
It's associated Stackoverflow post: https://stackoverflow.com/questions/74294256/h2o-python-relevel-vs-relevel-by-frequency-for-factor-columns

[maurever]

@hutch3232 Thanks a lot for reporting this bug. The fix is being reviewed and will be available in the next fix release.