There are multiple threads about this issue, and it was logged in the issue tracker back in '15. In the AnswerHub thread the issue was filed from, Rudy Q says it was resolved, but as you can see it’s actually marked Won’t Fix. Neither original post in that thread, nor the write-up in issue UE-23843, make it clear but additional comments in the thread do identify the problem:
When using the mobile Touch Interface, 3D widgets placed in screen space cannot be interacted with. If they are set to World Space they seem to work somewhat (I haven’t tested extensively since that’s not what I need); however, when set to Screen Space they don’t respond to input at all.
I have confirmed that turning off the Touch Interface resolves the problem; Screen Space 3D widgets work fine then.
So is there any way to get more insight into why the issue was marked as Won’t Fix? (the public issue tracker doesn’t show any comments / history as far as I can see)? Is there a known work-around or fix? Another approach to achieve the same effect?
The fact that the bug was open for over three years before finally being marked Won’t Fix doesn’t give me much hope… :-/
[PS: using 4.26, not 4.26 Preview, but that’s not yet an option in the version selection list…]