Weird auto-hiding vertices in weight painting

Hello, please help me solve this weird issue. I want to select vertices in Edit mode to paint them in Weight Paint mode with vertex masking on. However, whenever I switch to Weight Paint mode, The vertices I selected became hidden and vertices of another vertex group show up. If I go back to Edit mode, the vertices I want show up as half-faded mesh, and I need to to press Alt H to access the vertices even in vertex mode. Now, if I select the vertices and go back to weight paint mode again, they are automatically hidden again while unrelated vertex group show up. All vertex groups are unlocked.