Thanks for your post.
First, what version of FileMaker are you using?
I would recommend several things in this scenario. First try a differnt browser to verify the behavior is replicating in multiple browsers. Sometimes behavior like this is a failure of the browser to interpret the HTML code correctly. Asuming you get the same behavior in a different web browser proceed with the following.
Savve a compacted and cloned copy of the database. Compacting removes any unused data blocks in the database, which is the biggest source of corruption in a file. the cloned copy makes a complete structural duplicate of your database file without any records. I usally recommend the compacted copy first followed by a clone of the compacted copy. The compacted clone wil be the cleanest file you can get out of FileMaker. My thinking here is that you are seeing the byproduct of corruption within your file.
To make both of the above listed copies of your file the process is virtually identical. Save the file locally to a FileMaker Pro machine (if it's not already on the local drive). Open the file normally, then navigate to File>Save a copy as... and this will bring up a standard dialogue box for saving a file. At the bottom of the window will be a dropdown labeld Save A and list copy, compacted copy or clone. Make a compactred copy first, then open that copy of the file and do the same process choosing clone as the copy option.
Once you have both the compacted and compacted clone copies, host both through IWP and see if the behavior changes. If this fails recreate the layout object for the field in the compacted and compacted clone copies and see if this resolves the issue.
Please keep me posted wityh any results.
In addition to those tests, you might want to run a recover on the file an see if any problems are reported. I don't recommend using the recovered file if you have any reasonable alternative, but this can be a quick "cat scan" of your file for possible causes of this type of trouble.