I have had this problem too for a long time. Talked to Apple about it online but their solution was not to forward this to their developers for a bug fix, but to install the next update and see if this helps. So Apple's solution is to not do anything. Thanks for that.
The issue seems to be that the wifi assist only works on wifi signal strength and not on the ability to actually meaningfully transfer data to load even the most fundamental of website homepages. There is only one solution at the moment and that is that when you encounter these wifi networks that load nothing, that you have to go into your wifi option in settings and manually 'Forget this network'.
Not ideal but that's the only solution I have that seems to work.