Numeric tags

Hi
The tp_search box I have on my site searches content and tags of publications but ignores those tags that are numeric. All the publications on my site have a Research Project number, which is integers only, added as a tag, and visitors want to see the publications associated with a particular Research Project. Searching for tags as words works no problem. Searching for the project numbers brings up “Sorry, no entries matched your criteria” error. But in the backend database you can select the numeric tag in the taglist drop-down, limit selection, and it shows up fine. I don’t understand what’s going on there – can you help?

I have v5.0.17 of teachPress currently installed.
I am shortly going to upgrade everything to latest versions but want to solve this issue first.