Probably you are doing a lot of transactions and you only have one utxo. This is technical words, but try send money to yourself a few times.
We have a simple tool to improve this:
https://www.moneybutton.com/utxo
Please note that given when the button is showing a certain amount that money goes actually to yourself, so it's almost free. You only have to pay the fees for that tx.
Yes it is a bug. Money button should know when a transaction chain is nearing the current mining li.it and split the UTXO rather than issuing tools for users to do so manually after having the problem.
Users should not be required to fiddle with such things