SharePoint List field not saving, reverts to what was there before

Hello

I have a SharePoint list with a customised form. I am trying to update a text field that holds a URL

Field is typically hidden and is not in the customised form

When I updated it in grid view, it reverts back to the original content after about 5 seconds

I then added it to the custom form and once saved the original content of the field returned

Looking in the version history, i can see new versions but no changes underneath them

Any Help Appreciated

Conn