Yes, the camera will try to use the biggest possible aperture to ensure sufficient shutter speed.
But then it's a bit wierd why it doesn't go to F1.8 for your 50mm. Maybe you manually select a ISO that is high enough?
The logic is rather complex and not so simple as we thought
As example, my 70-300G run between f5.6-f7.1
My f1.7 go between f1.7-f2.2
My guess is sufficient shutter speed comes first, then followed by best contrast