To repair factions arties, you need to get the faction version of POF - Powder of Perseverance from the faction chest. Costs 300 silver and fully repairs the artifact. However, each time you use it, the max durability goes down by 25, and you can only repair it 5 times.
Faction arties lasts much shorter than replicas.
Replicas lasts a much long time as they can be repaired. I would advise against hoping you won't lose durability by regular repairs of the slightest damage.
In the long run, we tend to get owned by the RNG. I have reguarly gotten the reduced durability like what was experienced in the OP's firt post. ie Repairing 254/255 arties with a lengendary smith and getting back a 254/254 durability item...
Just accept that you will lose max durability and repair it only when it hits the single digits (or any higher number you are more cormfortable with).