Opening that test link you supplied, on Firefox running on a MacBook, it shows the same problem:

Same thing happens when I open the source for that page:

So it's not iOS 18.1.1. that is the problem, it is something on the server. Hope this helps.
Edit: I took the source code from the test file, replaced the diamonds with pound signs (not the HTML code for one) and have saved that as a HTML file using Dreamweaver. The result is here:
TEST
You can have a go and see how this displays on your devices.