Since it got stiff later rather than right away, I think we can rule out poor installation. Also that usually frees up with side pressure (flexing the chain sideways).

The two main other causes of stiff links are rust, and overly agressive shifting under load.

Rust can occur between the plates,without being obvious from the outside, when water wicks in . The riding conditions and your choice of chain lube are contributing factors. A rust bound link can usually be freed by forcing the chain to flex at the pin, the normal way (not sideways). I do this by sliding 2 screwdrivers into the chain on either side and using some brute force. Once the link is free, add a drop of oil and work the link back and forth until it's truly free. Once it's decent, riding will only make it better. Use decent chain lube to prevent a re-occurence.

Shifting under load and cause stiff links by peening a nick into the plates effectively making them thicker at the edge. The hard impact of the sprocket tooth on the edge of the plate is the same as if you put a chisel against it and hammered. This type of damage can be seen with the naked eye, and can be harder to fix. Sometimes repeated flexing will loosen it, but often you'll have to carefully file the nick down a bit. Again once it's decent enough to run OK it gets better over time as the mating surfaces wear. Prevent this by easing up on the pedal pressure as you shift.