You do have fish in this tank, right? If so, the nitrogen cycle has a by product of nitric acid that will gradually reduce your kH. You will need to do more than 40% water changes each week to reduce the nitric acid and keep your kH more constant. Yes the bogwood could be adding to this by leaching out tanic acid.
Up your change to 60% per week and see if that controls it.
If you are using a set point of pH 7.0 and controlling it with CO2, you are gradually reducing the amount of CO2 you are injecting as the kH goes down.
How are you controlling the pH. If with CO2 then see above. If with buffers, then you may be using a phosphate buffer which is why your PO4 stays where it is. Do you have PO4 in your tap water? That's the another likely source. The last source is from feeding your fish. With 40% water change, and not fully planted, it is unlikely that your PO4 will go down. It is more likely that it will go up.