Chart gridlines can use some really wacky intervals, and if you jiggle the chart width by just a few pixels, it can reset. As far as I can tell, there is no way to control them. I asked in the forums and got nothing, and have searched extensively through the controls in Pro, so I'm guessing I've taken this as far as I can.
Here is the question I asked with screenshots and such: https://community.esri.com/t5/arcgis-pro-questions/chart-grid-line-intervals/td-p/1018190
