As far as I know slower 787 climbs are due to RR engine issues and RR recommendation to use CLB2 whenever possible to prevent engine damage. Also, some operators modify climb derate washout to keep derated climb thrust until TOC. This generally results in slow-is climb rates especially in the mid20’s to low30s.
However, climbing at 348kt doesn’t make sense, I’d venture it was someone’s not-so-bright idea rather than airline SOP.