Thanks for reaching out.
The feature you are looking for is unavailable. I tried on the devices we have here and didn't find a setting to turn that feature off.
If you haven't already done so open biometrics and security/ other security settings and turn off make passwords visible. It shows password characters briefly as you type them. On my phone it does this but then replaces each character with a small white dot.
I'm thinking if it doesn't show the password characters at all then there shouldn't be any password characters to leave behind as a ghost image.
I also have set mine to confirm pin without tapping ok. This way it unlocks and loads the homscreen automatically as soon as I type in the last character.
Galaxy S9 SM-G960U
Security Patch Level
December 1st , 2019