I have the same problem and I discovered that it's a general problem of compatibility of using different clients. In my case all attachments added to a note on the android client (pdf, jpg, PNG) will not be shown on the note opened with windows client.
While in the other way it will work. Means if I start a new note on windows client, the added attachments are without failure on Android client and I can view them
This bug is really limiting me in my workflow