The only thing I've found is the 3rd party program solution. CoSign is the name of the product I see most often referenced. Mind you, I am looking on behalf of my users/customers who have documents with multiple signatures they've saved inthe past, but cannot do it today. It appears that either Microsoft (these are Excel and Word documents they are referring to) produced a security update that changed this, or a 3rd party program is no longer effectively working.
For reference, I've been actively searching the internet for days for an answer - unfortunately, I've only found 3rd party programs referenced. Which I could have been more help.