By what you described, I believe this is the same as this post: No player collision when player flight enabled
If that’s what you are experiencing, as that post above says, that’s an intended feature: you can enable phase while in edit mode to do debugging during gameplay and be able to phase between objects in the scene/world.
If you want to simulate normal gameplay with that behavior disabled, you just need to disable phase while in edit mode before starting game.
Keep in mind that this behavior only exists during edit sessions due to the debugging purposes, that behavior does not exist in playtest/private codes or published islands.