Editor support on mobile seems not fully tested. (I'm using Android 9 (Lineage OS), with Chrome as browser and Swiftkey as the keyboard)
This morning I tried editing an answer, but I got the new editor, which meant I couldn't select text and click the code button. When posting a comment I suddenly got the workaround (the old style editor with a preview window).
I tried setting the browser to desktop mode, but then all the admin tools disappeared, so I couldn't edit the answer anymore.