There used to be an issue with WordPress – but that should have been fixed 5 years ago: Preserve PNG transparency/alpha during thumbnail creation
If you’re using a newer WP installation (which I suppose) this issue might be rather a problem with the GD library of your server than with WordPress. And as far as I know this issues only occurs with 8bit pngs. Using 24bit pngs you should be on the safe side.
I found this ‘suggestion’ over at the wordpress support forum. However I can not confirm this ‘bug-fix’ myself since my webserver seems to render both 8 bit and 24 bit images just fine (tested here with WP 4.3, GD bundled 2.1.0 compatible, PHP Version 5.3).
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