Hi,
It is not really a bug.
But when a variant does not have a price, the price of the main product is used ; and "0" is used as an absence of price.
There is a way to specify a "0.0" price without letting HikaShop consider it as "empty" ; you have to set "-0.0" for the variant price.
In that case, it should work.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.