Seems like the tag panel wasn't meant to be used with more than 18 items.
The tag-panel window is not scrollable in horizontal direction when it has been adjusted (docked) to the left side of the screen.
When adjusting (docking) the tag-panel window to the upper side of the screen, then scrolling in vertical direction can show more edit fields, maximum maybe 115 fields.
This has nothing to do with scrolling. Look at how much white space is wasted, there's absolutely no need for scroll bars with that amount of items yet:
The issue here is that there's a hard limit of three columns no matter how many text fields there are and also that the first two columns have a hard limit of six items while the final third has no hard limit of lines at all.
Ideally the text field should auto-adjust depending on window width and the height of the horizontal seperator between the tag panel and the file list, or depending on the size of the free floating tag panel window if it is seperated. That is the expected GUI behavior. As an alternative - which is used by many GUIs - let the user decide the hard limit for the number of columns or the hard limit for the number of lines per column, so that going from maximize to windowed doesn't mess up the tag panel. For instance I'd like to group related items in their relevant columns together.
Well, already some time ago there was a discussion about too much space being wasted on the track field [when the Tag Panel is docked at the left side]; and the lack of ability to put for example DISC NUMBER tag field in the same line as YEAR / TRACK / GENRE
Looks a lot like a feature request to me.
Compliance with somebody's taste or the absence of the it is no bug I would say.