When using the Local | Clean Up dialog, there's an option to 'reset (expire) reflogs now'. If this option is selected, any stashes may be deleted.
As I found out to my detriment (I lost several weeks of work), this is normal git behavior. But my assumption when using any revision control GUI is that any operation that can destroy data will at least be guarded by a confirmation dialog.
As a suggestion, please consider a warning dialog in this case, to protect people from unwittingly losing their stashes.
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