Why bridges offline for a few days get first_seen reset to 1970-01-01

We kept this bridge out of bridge statuses for 24 hours https://metrics.torproject.org/rs.html#details/2D8C27AA9C2FEC3EC468D7ABE9C3EDCA3C86610A and the first_seen date has been reset to the epoch. We should find out why this is happening for bridges only.