Well, I'm not sure... This has been happening since I freshly installed the app after the iPad (I'm using an iPad Pro 2020 12.9") reset, right from the beginning of using the app. I think since the 4.0 update...?
I backed up the brushes and reinstalled the app and.... still doesn't work. All I did was change the settings and upload the brushes. I am attaching the videos. I don't do anything specific before the error occurs, I just open the document, draw and there it is. Literally. I use iCloud storage. I guess eyedropper sometimes doesn't seem to update the changes on the layer in real time (only in "current and below", for me at least). But I have no idea why or what specifically causes it...