Steps to reproduce:
1. Open attached project
2. Open any issued scenes (01IssueCollision2d)
3. Click the Play button and wait for a while, it pauses automatically
4. Press the Step button several times

Expected result: As soon as "Sphere" GameObject collides with "Cube" GameObject Unity should register a collision.
Actual result: As collision happens one frame passes, and only then Unity registers a collision.

Note: If you set "Sphere" RigidBody Collision Detection setting to "Continuous", then Unity detects a collision on the first frame. This issue does not occur on OnTriggerEnter2D.

Reproduced in 2017.4.12f1, 2018.1.9f2, 2018.2.11f1, 2018.3.0b5, 2019.1.0a4