Yup, that was it.
This is a result of the Forms preference "Show border hover color for fields".
The actual feature works like this: if the feature is set, all form fields when inactive will be displayed using Acrobat's default rendering of that field type on top of either the fields highlight color or the required fields highlight color. When the field becomes active, it will be rendered with the appearance stream in the widget annotation dictionary.
Talk about a total misfeature! It violates the principal of least astonishment - if I specify an appearance stream, Acrobat should use it to render the annotation. Acrobat should be able to figure out how to highlight it in an appropriate manner instead of substituting a different appearance stream which will likely look totally different from mine.