Hello again. If the dim digit still bothers you, the cause may be insufficient current running through the tube's filament and not the way the tube is driven. There might also be a straightforward solution: Replace R3 with a jumper. R3 is a bit difficult to remove, as it's squeezed between the alarm switch and VFD chip socket. So instead of removing R3, you could also short out R3 by soldering a jumper across R3 on the bottom side of the board, placing the jumper and R3 in parallel.
For a more technical discussion, see viewtopic.php?f=41&t=23586&start=21