I did clean it when I did the switch replacing. Sprayed the whole back of the PCB down with Electronic WD-40 (which I'm pretty sure is just aerosolized isopropyl alcohol), and the board is pretty immaculate.
What happened is one of the leads from the battery terminal (the left wire specifically) kept coming out because I could not get it to stick in the solder. Then when I tried to resolder it back in I ended up 'cooking' the end of the lead (as I said, soldering skills = abysmal) on the solder iron, so I had to clip off the burnt end and try again. After several iterations of this, when I did finally get it to stick in the solder again the wire was now stretched very tight and the connection was terrible, such that jostling it around even slightly caused the board to either not register at all or have 50% of the keys work.
Again, this is why I wish we had an Alps equivalent to Phosphor Glow so that those of us less-technically-inclined could just send our boards in to be renovated/fixed instead of having to do everything ourselves. It's the single biggest frustration of dealing with Alps boards, IMO.