This is not a bug. It’s how the program is currently designed.
When you are editing any transaction, the Amount shown on edit screen is not rounded but when posted to general ledger, it gets rounded (in this case) to 2 decimal places. This is necessary so your figures on reports don’t have more than 2 decimal places. You certainly don’t want your profit & loss statement to show 21.145 in some interest account.
This issue is not related to bank reconciliation. You can reproduce the same issue everywhere.
I want to improve edit screens so they round the figures in Amount field as you type so you can see the total for the transaction will be 42.30 if entered that way.
Just one thing I mention, using 0.5 qty is a clever trick but it won’t work on 42.29. It will work on 42.28 or 42.30.