Maybe a slight leakage of fuel from the positional thruster?
Forbush decrease effect? Although typically thought of as an indication of future solar activity, the Sun’s Great Conveyor Belt has slowed to a record-low crawl that’s considered “off the bottom of the charts.” A slow belt means lower solar activity. Less plasma circulation = less solar flares and radiation storms = less magnetic force fields and increased radiation acting upon spacecraft. It would be interesting to see the solar activity at the time of all referenced flybys.