These functions are accessed by multiple presses of the WIDE button or the Menu >> Settings >> Preferences >> Video
options. Presuming your TV is widescreen (16:9) and the screen ratio is set to 16:9 on the Humax, none of "Zoom", "Pillarbox", or "Auto" (the three options available) will have any effect on 16:9 source material.
If the material is 4:3 however, "Pillarbox" presents it as 4:3 (ie undistorted) with black regions to the left and right filling the 16:9 screen. "Auto" stretches it horizontally so that the original 4:3 frame now fills the 16:9 display (but everything is short and fat). "Zoom" expands the frame to fill the width of the screen, but cuts off the top and bottom.
Problems sometimes occur when a broadcast is 4:3 material intercut with 16:9 ad breaks. The codes sent to switch between the formats are either not transmitted correctly or are missed at the receiver, and then either the 4:3 content or the 16:9 content can be mis-displayed.
However, as you are fixing your particular problem by switching the TV rather than the Humax, I suggest the problem lies with the TV (confirmed if the WIDE options do not resolve it) - or it is simply a format switching error between ads and programme as noted before (and the HDMI resync forces a reassessment). Note that cycling the VFORMAT also forces an HDMI resync.
See also HERE (click)