I've added a Column Validation rule in a modern list in SharePoint Online. This works as expected when creating a new item - if the validation fails, the save operation is cancelled and the error message is displayed under the field and at the top of the form.
However when I edit the form, the validation message displays for a fraction of a second under the field and then disappears. No error is shown at the top of the form.
In view mode, I have experienced different issues. This mode now allows you to edit individual fields. If there is an invalid value in one of the fields, I can sometimes change another field. On exit of the field, it saves the form and then briefly shows a validation warning. The warning is only displayed for a fraction of a second. At other times, I cannot modify any field, it always flips back to the field with the validation issue, but again, only flashes the validation message.
I'm using the latest version of Google Chrome. I've tested on two separate tenants and seen the same experience.