Adding Signature get "Annotation Modified"

The checks in Adobe Acrobat for changes to signed PDFs have increased more and more in the recent years to counter a growing number of techniques to apply unwanted changes to a signed PDF that the Acrobat versions of their time did not report.

Not all of these techniques work by appending something obscure to the PDF after signing in which Adobe Acrobat fails to determine some actual change. Some techniques also work by preparing a PDF before signing in a way to make Adobe Acrobat  display the document differently after a signature is added. Thus, these checks do not merely check additions after the signed revision, they also look for suspicious content already present before signing.

At the beginning of this year there was yet another update of these checks which already was the topic of other threads here. Apparently this update also caused your issue.

I've had a look at the internals of your file but I couldn't determine what detail makes Adobe Acrobat report that modification. There is an error in the annotation, the IT intent entry is FreeTextTypewriter but correctly should have been FreeTextTypeWriter; but fixing this doesn't change the warning. Apparently, therefore, there is yet another suspicious detail that didn't catch my eye.