Thank you for your post.
Is the file local to the iPad, or are you accessing a hosted file?
To make sure the signature is being saved, skip to the previous record and back again. Does the signature still appear?
Does anyone else have access to the iPad and possibly deleting the contents of the Container field?
Are you noticing if this is occurring with only certain records?
Any other information you can provide may be helpful in determining why the data is being lost.
If this is from a hosted file, might the container field have global storage enabled?