Entering MPN in ME51N only possible through F4?

Hi

I have set up the config to allow the MPN to be entered in ME51N. When

I use F4 lookup and select the MPN from this selection, the field holds

this material number. However, if I enter the material number manually

(directly into the field without using F4), then the field is not

populated with the material number.

I would have expected entering the material number or using the drop down list would work the same way. Has anyone ese come across this issue?

Regards