MDP warnings are related to the modification detection and prevention capabilities of a form. MDP is typically used when there are multiple signers like
fill1 -> sign1 -> sign2
fill1 -> sign1 ->fill2 -> sign2
The publisher of the form makes decisions on how MDP gets configured, while creating/programming the blank form. This is designed to associate specific fields of a form to specific signers.
First question - where did you obtain the form? It's possible it could have been corrupted. For the DD2656, we do see an official version on this portal: https://www.esd.whs.mil/Directives/forms/dd2500_2999/ and more specifically at: https://www.esd.whs.mil/Portals/54/Documents/DD/forms/dd/dd2656static.PDF
Is that where you received the form? If you received the form from another source, does the above work better? Also make sure it is a clean/blank version of the form downloaded and saved with a new file name on your local computer, and not one that has been previously filled in and attempted to be re-used/changed.
Second question - what version of Acrobat/Reader do you have installed? Note that 21.01.20039 is current as of this month. Information on updates is available here: https://helpx.adobe.com/acrobat/kb/install-updates-reader-acrobat.html
Third question - what CAC/PIV/PKI middleware software and version do you have installed, or are you using the native Windows driver for your card and reader? Is it working to sign other PDF forms OK?
Last question - we do not need to see anyone's personal data on the form, but it would be helpful for us to recreate the error by knowing which form sections were completed with any data. e.g. 1-5, 6b, 7a, 8a, 9a-g, etc - then we can complete the same fields on the same form using generic test data, and attempt signatures. It would also be helpful to know if the error is occurring on the very first digital signature applied, like field 39b, or the second signature field like 40b, etc. That helps determine whether there is a form bug before the first signature, or between the first signature and any subsequent counter signatures.
This detail will help determine whether there is a form issue, an Acrobat application issue, and/or some other configuration issue going on and be able to make further recommendations. Please keep us posted.