Here is another translated slide from the JAXA SLIM briefing.
It states that the main engine was operated at higher load during 8 orbit changes and during the power descent phase, but there were no signs of abnormality in the main engine until the event occurred.
The the -X side (failed) engine had rather mild combustion conditions compared to the +X side engine.
The team believes that some external factor other than the main engine affected the the -X side main engine.