I am currently facing the problem, a snippet is inserting the current content of my clipboard instead of its actual purpose inserting the current date and time. It could be a problem with macOS 10.14 Mojave because it seems to exist just since then, but had worked well before updating to Mojave.
OK, it seems that this only happens sometimes in a text field for the document name in DevonThink Pro Office. This is what I made this snippet for. It dosen't happen in other text fields (multiple or single text line) like in a form of a website in Firefox or Safari. Also no trouble in different areas of OneNote.