I notice this in polylang only with a few posts, out of over 100 that can find the existing translated page easily. These are often posts that have short or generic titles, that don’t come up easily in search either. Sadly this isn’t an answer, but to me it seems related to title, rather than to a 3rd party plugin. I will probably do some more investigating around this because it’s been a persnickety problem for a while, and the stock solution doesn’t seem to help.