I can reproduce the bug, but only after I slightly tweaked the sample code..
I added in a text box between the 2 buttons.
To replicate the issue...
If you sign the top signature box, then tap into the text box, it clears the signature box.
In real life, we have customers signing, then typing in names before saving, hence the issue.
I think what's happening is that the new version of Appstudio is auto clearing the signature when it passes focus to a text box.
Can you confirm you can replicate this?