An update is available that fixes the following issue:
Fixed a bug affecting the paintings with several missing objects
If the player placed at least one of the missing objects in the painting and then left the game, the painting was no longer unlocked if the player placed the remaining elements later. IMPORTANT: If you have encountered this bug before downloading the update, you must manually fix your game save. To do that, simply press the "ctrl" and "D" keys on your keyboard at the same time while playing the game. The paintings in question will be unlocked. Sorry for the inconvenience. Thanks to Innocentive and Karlusha1029 for reporting this bug.