This could be due to CTA Button images being scaled based on the CTA Font Size variable.
Here are the steps to fix it:
First, input the CTA Text for the button as if you were using this feature (End Card → Call To Action).
Next, scale the text to the size that you want the button to be (End Card Call To Action Font Size).
Lastly set the CTA text to transparent so it doesn't interfere with the image text (End Card Call To Action Font Color).