That's a common bug
One that I thought I had fixed by now, but it rears its ugly head again
Until I figure it out,
stretch the window, make it wider
That's the easiest fix
otherwise, this can happen when you have too many tags in total
Magenta_Ivy made a modified version of your Pack Editor for his mod. It's a bit older version but you could use it for a basis for this problem. (Here the tags are on the side not below). At your version i cant even see the tags. Here is a link for the modified version: https://www.mediafire.com/file/12h0vip3pwbxnba/PortraitPackEditor_2.16_Modified....