Yes, how to clear cache on an iOS app is the question. And yes, no deletion possible from Olas itself, only from Amethyst where it's now gone already for me 😄
Discussion
https://www.capterra.com/resources/how-to-clear-app-cache-on-iphone/
In the end, I deleted Olas and reinstalled in Testflight. And then voilà indeed, the media note I deleted from Amethyst also disappeared on Olas 👌🏻