The difference in translations depend on whether the negative וְאַל (= and not) is considered extended to the following verb וִיהִ֥י (= let be).
Note the suggestion of the Pulpit commentary:
Verse 6. - And let not his men be few. The negative, though not
expressed in the Hebrew, is to be carried into this clause from the
Ellicott is similar:
It seems best, therefore, to take the whole verse as applying to
Reuben, and the negative in the first clause as covering the second
clause also. “Let not his men be a (small) number.”
The versions are divided as whether they agree as noted by the OP. Literally, the Hebrew reads:
Let Reuben live and not die and let his men be few.
However, if we allow the force of the negative in the first clause to apply to the second clause (as noted above), then the verse would read:
Let Reuben live and not die and let not his men be few.
As Ellicott notes:
The most terrible destruction ever wrought in Israel by the word of
Moses came on Dathan and Abiram (who were Reubenites), when “they and
all that appertained to them went down alive into the pit.” We cannot
say how far the tribe was diminished by this terrible visitation and
the plague that followed (Numbers 16), but the fighting men of the
tribe had slightly decreased in the second census (Numbers 1:21;
Numbers 26:7), and only two of all the twelve tribes had a smaller
force than Reuben at this time.