Video reference:
It will be easier to see the blue line if the video is full-screen.
(If the video player doesn't load: https://youtu.be/EUgaWhvBWq8.)
When I start up my game's preview, there is a "blink and you miss it" flicker of a blue line on what could be described as the "horizon" of the layout with the 3D camera being utilized.
I've had this problem before but couldn't identify its cause. One thing I can confirm through prior testing is that this line isn't actually "in the distance" as it always appears in front of other objects regardless of their Z Height and XY distance from the Camera. So, it seems to be more of a visual "tick". I just don't know what's causing it or how to fix it.
Here is a link to my current project as it is now: https://drive.google.com/file/d/1Sg3iKvcTe7-oL4_H-mEdtvIMj_Lov4Qk/view?usp=sharing.
The Camera_Base object is what I'm using to better manipulate the position of the 3D Camera object. Its setup starts at Event Line 29. I believe that's all that is relevant in the event sheet.
I also just switched the Z Axis Scale value in the project properties window from "Normalized" to "Regular". I did this because I wanted to be able to better match the Z Height values to the Width/Height values of certain objects and I figured it would be easier to do this than to guess what value on a 0-100 scale works with the sizes on the regular XY scale.
I hope that helps narrow down the details. Thanks for any help you might be able to offer.