I have found the legend fitting strategies in ArcGIS Pro to be quite unreliable. Specifically, I often experience shifting columns when using the "Adjust Frame" strategy with a Map Series. My understanding with the "Adjust Frame" setting is that the legend will never automatically manipulate the number of columns that are shown in a legend, but rather change the size of the frame that the legend occupies. In this way, I would theoretically be able to orient a single legend as follows, and have it maintain this orientation.
Here I have "Adjust Frame" set, both group layers (Heading, Group #) are checked as "Keep in a single column," and "INDIVIDUAL LYR 2" is checked to start a "New column." This is exactly what I would expect. However, when clicking over to the next page in the Map Series, and then clicking back, this happens. No changes have taken place other than clicking "next page", then back a page.
I find this sort of thing happens constantly--ArcGIS Pro legends will not reliably respect "New column" and "Keep in a single column" designations when using the "Adjust frame" setting. I find that shifting like this occurs if I change the font size of a patch, change item gaps, add another patch, etc. If the global setting was "Adjust columns," that would be fine. But it's not.
This particular example is happening when I switch pages while "Only show features visible in the map extent" is checked. If the number of patches to be shown on a given sheet is more than the examples above, all different kinds of arrangements can show up. There's no consistency at all from what I can tell. The goal is maintain two columns with the underlined layers at the top of each, followed by however many patches need to be shown on for the given sheet in the group layers below.
This was a trivial setting in ArcMap, so I assume I'm missing something in Pro, but it's not obvious to me. Any thoughts would be greatly appreciated.