1230 items in a single check box field? !! That's an extremely large number of values for a single field. You can split your value list into several lists that when combined make up your current 1230 item list. Then you can place several copies of the layout on your field, each formatted with a different subset of the entire list of values. This can look and function like your original layout design and still enters all values into the same field breaking up the list into sections like this may enable you to get it to work in IWP.
But you might also consider a more basic redesign of your table structure. Such a huge number of items in a single field does not seem like a very practical way to store this data.
Even when using a smaller value list (217) there are still 8 items at the bottom of every column that are getting cutoff. I'm thinking it may be a formatting issue.
I should have mentioned from the start: I inherited this project, and am still learning the software. I can just find very little information on formatting errors in IWP.
When using Chrome, zooming in to 110% shows the bottom of the field data. No other web browser seems to work with it though, even when zooming.
This seems like I'm either missing something simple in my layout options, or there is an issue with the IWP formatting.
I will repeat my recommendation that putting over a thousand items in one field is not good database design. THere are alternatives that will work much better.
Even 217 items is a lot to put in one copy of the field.