I find this an extremely annoying bug and I have seen other players comment about it on the Steam discussion for Stardew Valley. When you acquire enough foraging skill to reach the next level you immediately revert to level 0 foraging skill with regards to tree seeds for the rest of that day. This means that trees no longer drop seeds if shaken or if cut down. Also, if you hit a seed on the ground with your axe or pick the seed is destroyed as if you had 0 skill in foraging. This will reset once you go to sleep, but for the rest of that day where you gained the skill to reach the next level tree seeds are broken. For example, in my current game I am level 3 in foraging. I need to cut down 4 more trees to gain level 4 in foraging. On this particular day I need 350 wood for some crafting projects. But I can't reach that goal because as soon as I cut down 4 trees and reach level 4 in foraging I can no longer craft field snacks as no more seeds drop. Without the energy from the field snacks I can't cut down more trees even though I have all day to it. Quite annoying.