Got a reply from support today...
As far as I am aware, the User Media plugin is working.
Doing some tests according to your message, it indeed appears there is an issue, probably due to the last iOS update.
In regards to issues with Construct 3, please visit our bug database and make sure your bug was not already reported.
If it was, you may add informations as comment to the ticket.
Otherwise create a new report and fill all the informations required to allow our devs to investigate in order to fix in the next releases of Construct 3.
Apparently there was no issue opened.
I've made a report : github.com/Scirra/Construct-3-bugs/issues/1964
Consider adding any further information/project related to the issue that you may have and follow up on any test or further investigation our devs may require.