In the Gryffon Castle there is a button which activates only if you press down the two pressure plates before it with 30 kilos each.
The bug is, there's no indication you should do this, as neither visualy change anything, nor you can hear any sound. But the solution works.

I wouldn't call this a bug. There are various puzzles throught the game with the pressure plates and none of them have any indication, because they're puzzles that you need to figure out. That's why it's really useful to carry a bag with 10 rocks in it - different pressure plates require various weight. This is also safer, since the number of available block figurines is limited and the bag can be used multiple times.

All pressure plates when activated sink in the floor which is also signed with a sound effect.

Btw, I have so many stuff on me, I barely can carry any rock around if I don't go overweght (I have 8 agility and 6 strength for better chance to hit). I usualy use spare weapons and whatnot to weight down plates.

And when it does not sink into the floor that means it requires either more weight, or something else (like a switch for example) to do so.

You can put the bag with rocks at the beginning of each dungeon, so that you don't need to carry it with you all the time and get exhausted due to having too much stuff on you, and only pick it up when you actually need it. The downside is that you have to remember about it, when leaving a location. It will not disappear, but it may take a while to backtrack.

But it DOES think, except there is no visual or audio confirmation of it.