Problem with too-high logic level / floating signal in 15 x 5 keyboard matrix
Some keypads don't even use diodes. But you get ghosting then. So diodes are added. But the reason they are added is to isolate one switch from another. You've got the diodes chained in series.

Let me illustrate with an example similar to what you presented, using four switches, but done correctly:







simulate this circuit &ndash; Schematic created using CircuitLab

It should look more like that. Note that the diodes don't form a conductive path that proceeds through more than one of them at a time.