Thank you for the fast response.
I have tried to install yEd on several machines at work and they all have the same display issue. There are no custom themes, they all use the recommended 150% scaling. The only thing that is “special” is that they are accessed through Remote Desktop. I’ve been working remotely for the past 2 years, and I never had issues with that but anything is possible.
As an additional test, I installed yEd on my corporate laptop and indeed it does works fine on a 4K monitor. It just doesn't work on the computer that I need to use!
Any suggestions are welcome, thanks for your help.