Same here, on linux and mac.
Ctrl-Tab/Ctrl-PdDn goes through the tabs in the order in which they are listed in the drop-down menu on the top right, NOT in the order in which they appear on the screen.
Ctrl-Shift-Tab/Ctrl-PgUp go in the opposite direction, i.e., these keys are consistent with each other and with the tab list in the menu, but not with the screen.
This bug was also reported in Why does the shortcut next and back tab not work like chrome but that has been closed.