i wouldn't be bouncing them off any surface. they are taken one by one out of the bag and placed carefully onto a surface not bounced.
They are bouncing off each other in the bag. If a dice is weighted to roll a 6 more frequently than it should otherwise, it doesn't matter if you are bouncing it off the floor, a table, the inside of a cup, other dice in a bag, dropping it down some stairs, or launching it in a trebuchet - it will still be more likely to roll a 6.
Neither their Android nor their Apple apps are reproducible from their published code:
https://walletscrutiny.com/android/com.cakewallet.cake_wallet/https://walletscrutiny.com/iphone/com.fotolockr.cakewallet/Still, that error is horrendous. They are falling back on a function which the documentation specifically says is not suitable for cryptographic purposes, which apparently also defaults to a 64 bit number:
https://devoncarew.github.io/papyrus.dart/dart.math.html#RandomCompletely amateur mistake. Yet another reason that people should stop using all these random wallets which keep popping up and just stick to the tried and tested ones.