Upscaling 4k Glitch artifact

Hello,
I'm trying to upscaling old video and I'm facing trouble in exporting the video:
If I upscale to 2k, no problem. If I do it in 4k, there's compression artifact, as show on this caption

Most of the time, these problems occurs on a black screen after a simple cut sequence.
Is there a 100% safe way to avoid these glitch?
(Sorry if there's english mistakes, I'm french and my english is a bit rusty.)
I'm trying to upscaling old video and I'm facing trouble in exporting the video:
If I upscale to 2k, no problem. If I do it in 4k, there's compression artifact, as show on this caption

Most of the time, these problems occurs on a black screen after a simple cut sequence.
Is there a 100% safe way to avoid these glitch?
(Sorry if there's english mistakes, I'm french and my english is a bit rusty.)