Bonus chest frequently spawns in mid-air or on top of water block. The following seeds repeatedly produced this result but not always in exactly the same location or at the same height above ground:

2000229013

2000229014

Links to screenshots:

https://drive.google.com/file/d/103Vv7MgMsWicmUhn_BON1wtf3HL2RjQ_/view?usp=sharing

https://drive.google.com/open?id=1kiUZtGefd1fxEfE6MC5sSCt8md_hIXer