Hey there again, Maaxxx!
Our team investigated the issue (even though it is not related to just our game, but every game made in the Unity engine) and we may have some insights into what's causing it;
Try the following:
If you've got an AMD Graphics Cards, go to AMD Radeon settings and under 'display' change scaling mode from 'preserve aspect ratio' to 'full panel'.
If you have a NVIDIA Graphics Cards, go to Nvidia Control Panel, and change "Aspect ratio" to "No scaling" on the Adjust desktop size and position page (with "perform scaling on Display").
Please, do tell us if this changes fixed the issue. Our main goal is for everyone to be able to enjoy Heartless Lullaby, it would be a shame if something like this kept you (or anyone else) from playing the game!
Cheers,
Entalto Studios