No corruption, this is a "feature" of filemaker pro text fields. You can edit the text to assign fonts and styles to any portion of the text in a text field you wish. If you copy and paste text into a text field, the font and style is copied and pasted along with the text.
Many folks apply the following auto-enter calculation to fields where text may be pasted from other sources:
If you use this calculation, make sure you clear the "Do not replace existing value..." check box.