Steps to replicate: On Windows, 3 monitors (SDV is on primary monitor in center), windowed-borderless, use Ctrl+Alt+PrtScn to screenshot the active window.
What Happens: Gives a screenshot of an old state of the game - not what is currently displayed.
Current Workaround:
If I Alt+Tab to change windows in&out of the game before Ctrl+Alt+PrtScn I get a screenshot of the current game state (correct).
Something is getting updated when Alt+Tab'ing that gives the correct screenshot, that is not updating regularly.
I hope this helps, if you need more information, I suck at responding to email but I joined the Stardew Valley discord: Randy1672 and feel free to ping/dm
Example:
This is the image I got (old game state):
This is the image I got after Alt+Tab out of the game and back (correct & current game state):