Since the introduction of iOS 13.3 Beta on my iPhone 7 Safari browser no longer offers me the option to open M-Forums app.
This is a little odd because my iPad 6 running iPadOS 13.3 Beta now does, whereas it didn't before.
See image below;


For some reason this exact same function with the same email client and browser no longer works on the iPhone.
I know that this is related to "Universal Links" which has been with us since 2009 so it should work and does with Facebook, Instagram etc.

So this is the sequence:
I launch Spark (Email Client)
I click on a link to a thread in Mac Forums that I'm following.
Safari opens and goes to the comment referred to in the email.
I scroll to the top of the page and I see the panel (in the attached image) offering me the option to open this page in the M-Forums app.
Clicking that takes me to the same post/thread in the M-Forums app.

I've tried clearing cookies and browsing history in Safari and checked all my preferences are the same in everything to no avail
The annoying part of this is I really want this feature to work on the iPhone due to the small screen size rather than on the iPad where I could and often do simply use the web page to read and reply to posts.

If anyone has a hint on what I might be doing wrong I'd appreciate the feedback. Bearing in mind that I am running a beta iOS which could be the problem in itself. I have posted this to Apple as well.