Strange library filtering with Pillar commentary series

I have the Pillar commentary series and if I type "pillar" into the library filter box, all the relevant resources are shown including the one on John's Gospel and the one on his letters.

i get the same result if I use the "pntc" filter.

if I use "pillar John" I get both John volumes, but if i use "pntc John" I get the volume on the letters but not the one on the gospel.

With both pntc and pillar being recognised filters and "John" working as expected when used with "pillar" why doesn't it work with "pntc"?

i am not at my main computer at the moment so can't post screenshots. Can anyone else reproduce this?

this change seems to have just happened - previously the "pntc John" filter was working, There have just been some updates to these volumes and I wonder if that is related to what I am seeing.

Thanks, Graham