Hi Peter,
Good day to you.
We have checked the reported issue in our end with the provided code. We have set target framework as 4.6 for httpRuntime in Web.config as mentioned in your update. But validation works fine in our end. We have attached a sample based on your code in the below link
Please check the above sample and get back to us with the following details to proceed further.
1) Whether you get any console errors?
2) Product version in which you get this issue.
3) If possible, modify the above sample to replicate the issue else provide a video on the issue.
Regards,
Keerthana.