Just another idea to try to force a footer to stay at the bottom.
I was only able to make the footer stay on the flowed page by thinking about it a little differently. I have only done this with a single page document.
I build all of my flowed documents in tables, as many as needed with varying rows and columns as i need for 'positioning' my objects. I have of course used repeating rows in my tables with add and remove row buttons.
I wanted to have a footer but how to make it stay at the bottom? What i actually did is add an colourless table above my signature table but below my main table. One that is invisible on the document. It took a bit of messing around with the sizing of the rows and amount of rows but you will need to change that for your needs.
The basic premise is that when I add a row in the main table, the button also removes a row in the 'invisible' table. When i remove a row in the main table, a row is added in the 'invisible' table. As the 'invisible' table is between the main table and the signature table, the effect is that the signature table does not move.
You need to limit the max rows for the main and 'invisible' table. If you dont, you can just keep adding and the signature table will move down.
You can see the 'invisible' table in my example below just for tutorial purposes.
Example: I have two masters and one page with a very long table in it (to force it to flow for testing)
Set Page one to Flowed
Change the pagination
What this will do is: Page one starts with the NormalPage master and then will move to the FooterPage master. However, in testing my table spanned a page and a half and it created a third page with the footer on it. Which is not ideal.
When the Place field is set to the Footer page, you can see below how the spanning table crosses pages but not over the footer
If the form is set to flowable, it should just push down the Signature subform as the form expands.
I always create my flowable forms in tables, it enables me to position objects where i want through the use of multiple tables with varying amounts of columns and rows. When the table about resizes, it pushes down the table below.
If you placed the subform on the last page's master page, that could work but if you have flowing objects above it might change which page is the last page when they expand. I have placed items in the master page before and adjusted the content area so flowable objects can actually flow past without overwriting the placed objects. Flowing would be a better option here.