Hi Willem,
Thank you very much for all the details, I managed to resolve the issue as it seems.
There was indeed an indexing failure with some parts of the text – it was caused by a HTML tag stripping mechanism. In general it is not simple to properly strip HTML tags from a given string, and false positive matches can make things even worse. The missing texts were caused by mismatched HTML tags, but I found a way to deal with them by making a few direct modifications to the plugin code. It should be okay now.
If it seemingly works all right, I will make sure to include this fix in the next upcoming release for sure.