it is hard to be sure without seeing concrete details (e.g. mdadm --detail output, or cat /proc/mdstat) but I would guess that do of the new drives suffered some sort of error while new data was being written to them. So your RAID6 now is doubly-degraded. i.e. the data is safe, but another disk failure will cause loss of data.
You should wait until the reshape completes (if it hasn't already) then add two known-good drives. If you think the too failed drives are actually good, you should test them well before adding them to the array. Maybe look for error messages in the kernel logs to see what so of error occurred on them.