Whenever I use a whetstone it increases the amount of material drop % and time, as it's intended. But whenever I use 2 whetstones I don't get any drops whatsoever (from the hills giant at least). I think this happens because the drop % increase stacks and that the max drop % is either 100 or the standard 8 bits (128) and the stacking then causes the % to overflow into the negative % as bit overloading does with java. It could be because of different reasons but this is my main theory. This would mean that if you use 3 whetstones you get drops for the first 2.5 minutes, none the following 2.5 minutes, and nearly 100% drop rate the last 2.5 minutes (on hills giants).
I think you should fix this by increasing max drop % so that you could get multiple drops for 1 kill, but it's most likely easier to just stop the code from letting the stacking happen.
On Android, latest game version.