I have been surprised at times how some of the cheapest racks have some of the greatest claims for capacity. But the expensive racks from a manufacturer that has a reputation to maintain often have realistic ratings.

Thorn sells a rack that is rated for 40kg if you use M5 bolts. But they raise that rating to 60kg if you use M6 bolts. In their experience, the bolts to hold the rack to the frame are the limiting factor on their rack. They also caution that if you are riding on rough terrain in S America, cut their rating in half. I would trust their rack for those weights.
https://www.sjscycles.co.uk/carriers...ck-powdercoat/

Although I have two Thorn bikes, I do not have that rack. I use a different rack on their bikes that has a lower pannier mounting position, the Tubus Logo EVO, my 10 year old Logo is rated at 40kg. On one of my bikes I use M6 bolts with my Logo.

I would not trust that rack of yours with 50kg and I would be a bit nervous about hitting a solid bump with 30 kg on the rack. But if your pack is 30 liters, you probably only rarely have more than 15 kg on the rack.