Field property "ShowInEditForm" does not work in modern mode

Finally received a reply from Microsoft - this is expected behavior in modern experience:

"Thank you for contacting Microsoft regarding your request of “ShowInEditForm not working properly in Modern UI  Field is hidden in Display form as well”.

We have conducted a thorough investigation into this matter and determined that under the current configuration, the product behaves as expected.

The reason for the behavior is that in Modern UI the display form is at the same time an Edit Form ( it allows for inline edits), therefore the product correctly hides the field that should not be edited. We are looking into options of improving the user experience in this area but we cannot commit to a specific timeframe nor can we communicate any specific implementation details at this point.

For each request, we strive to find the best possible solution for our customers while maintaining or improving the quality of our product and ensuring continued integrity of the product code base. We thank you for bringing this issue to our attention and continuing to work with us on improving our products for customers worldwide."