I don't have a definative reason for this, but if it something that has afflicted the odd 105 and 116 cars for years. The fork can wear significantly at the contact patch that locates the gear in position. I would build up this point with braze and reshape the fork to suit. I have had lots of each type and most fix it, some don't. I never in years of working on both types of trannies (no, not that type of tranny!) worked ot a reason that some cars develop this fault and some don't. Could be backing up steep hills regularly. I don't think it is anything to do with the mounts or linkages (116) initially, but may be all to do with gears or useage.
If I was overhauling a transmission and the owner mentioned the problem, I would look for a better gear set in my pile of spares, but usually I would only make the effort this for my own cars. I would always check for wear on the fork.
You see, as the reverse gears are straight cut, sliding mesh unlike the forward gears that are constant mesh and rely on the syncro sleeve for engagement (no axial thrust on the selector fork once in gear). The reverse gear relies on no axial thrust while they are moving. The only thing that keeps the gear in mesh is the dedent ball on the selector shaft and the fork to hold the gear in the right spot. Once it starts happening, you if you don't fix it it it will probably get worse. My advice is, hold the lever in reverse when backing until you have the trans pulled down.
It is worth checking that the lever mount is located correctly. Too far either way can prevent the lever getting full engagement either end of the shift spectrum. An 85 model should be an Isostatic car, but I don't think that makes any real difference.