Suggestion.
Go "Preferences" and see "Keymap".

"3D View" -> "3D View (Global)" -> "View Navigation (Walk/Fly)"
set to "Mouse" | "Middle Mouse" | "Press" (or "Click Drag")
then
"3D View" -> "View3D Walk Modal" -> "Confirm"
set to "Mouse" | "Middle Mouse" | "Release"

Worked version 2.91.