Why is this happening?

Preview :

Compiled Build :

This is an issue with the Tilemap object as seen in the manual under tilemap:

https://www.scirra.com/manual/172/tilemap

"When inserting the Tilemap object, Construct 2 may prompt you to use the recommended settings for tile based games. These settings are Point sampling, Pixel rounding enabled and Letterbox integer scale fullscreen mode. It is highly recommended to use these settings to avoid the appearance of seams, which can make thin gaps or lines appear between or along tiles. Seams occur due to fractional positioning or scaling of the display...."

This is an issue with the Tilemap object as seen in the manual under tilemap:"When inserting the Tilemap object, Construct 2 may prompt you to use the recommended settings for tile based games. These settings are Point sampling, Pixel rounding enabled and Letterbox integer scale fullscreen mode. It is highly recommended to use these settings to avoid the appearance of seams, which can make thin gaps or lines appear between or along tiles. Seams occur due to fractional positioning or scaling of the display...."

I'm not using a tilemap. It's a sprite object, with each tile set to a different frame of an animation.

Oh. I don't know then why it is doing that. Sorry.

bump

Have you tried Project settings: point sampling, pixel rounding, letterbox integer scale.