The problem exhibited when mode selections were opposite your test.
I selected the normal default view modes for the the People and Relationships categories: the Grouped by Surname person list and the original Relationship view mode.
CardView defaults to being a Navigator when clicking an object. (An opposed to the rest of Gramps where the mousing is mostly about merely shifting the Active focus within the Category or opening an object editor.) CardView mouse click move to the Category of the clicked object and make that object active. The original navigation was always to the CardView view mode. @cdhorn later refined that Initial CardView inter‐category navigation would go to the CardView view mode. After being initialized, CardView remembered the user’s last view mode in that Category. (I find the Category switching too “twitchy” in the Relationships CardView. Most of my Gramps time is spent in the default Relationships view mode. Most of my edits, object adding and tree expansion is done from that screen. So it is disorienting when a twitch on the mouse teleports me to another Category.)
But with both CardView and FTV installed, every time a Navigator sidebar Category was clicked, the CardView mode activated. (I use the “Drop-Down” navigator sidebar and “hide” the default “Category” and “Expander” sidebars with the Plugin Manager Enhanced.)
I think that only you, @cdhorn and @kku (Simple sidebar) are doing much Navigator sidebar modifying behavior. Each doing entirely different things to the Navigator: re‐ordering view modes ; mouse alternative to its use, addon Navigator mode. Maybe you 3 could have a side discussion about interactions and avoidance of stepping on each other? Then inform @Nick-Hall if you determine the core needs some isolating changes to prevent unintentional Navigator GUI/event hijacking