When you have open midi editor and select different midi item in arrange view it changes 'Hide unused note rows'/'show all note rows' mode. Selected item inherits previous mode.
Location: Near Cambridge UK and Near Questembert, France
Posts: 22,754
Based on your three "bug" posts here, can I suggest you do a new portable install and see if this all goes away when you are NOT using your current reaper.ini setup?