This was still a problem in the early Space Shuttle program. An anomaly report from the second shuttle flight describes it:
The 1982 press reference manual describes the H2 separator and shows a single H2 separator in the fuel cell product water line.
Potable water is generated by the three fuel cells at a maximum of
11.34 kilograms (25 lbs) per hour. The hydrogen- enriched water from the fuel cell passes through a hydrogen (H2) separator, where 95
percent of the excess hydrogen is removed. The H2 separator consists
of a matrix of silver palladium tubes which have an affinity for H2.
The H2 is dumped overboard through a vacuum vent.
In the 1988 revision of the press reference manual, a second H2 separator had been added (emphasis mine).
The hydrogen-enriched water from the fuel cell power plants that flows
through the single water relief panel to potable tank A passes through
two hydrogen separators, where 85 percent of the excess hydrogen is
removed. The hydrogen separators consist of a matrix of silver
palladium tubes, which have an affinity for hydrogen. The hydrogen is
dumped overboard through a vacuum vent.
I note that the claimed percentage removal has been reduced as well.