Figure 4. Timeline of Recent OGA Events
I will link to two papers that I believe will answer your question, but basically it seems to me that there were recurring failures. The picture is from both papers. The timeline say the Sabatier was removed and returned at 11/28/2017 for TT&E.
2019 - Advanced Oxygen Generation Assembly for Exploration Missions - Kevin Takada et al https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20190030425.pdf
2018 - Oxygen Generation Assembly Design for Exploration Missions - Kevin Takada et al https://ttu-ir.tdl.org/bitstream/handle/2346/74107/ICES_2018_113.pdf?sequence=1&isAllowed=y