Playing the guitar or any musical instrument applies tactile sensations and this is the way the player memorizes where the fingers go. The solution is simple. Practice standing and playing until the physical responses become automatic as they are when you are seated.

Memorizing music has an order.

1. Tactile memory
2. Auditory cues
3. Visual cues.

If you are a singer, you are always singing to people because you want to communicate
the song, having nothing to do with being seated or standing.