Per - https://amp.dev/documentation/components/amp-form/#not-allowed - the form attribute seems to be restricted

<input type=button>, <input type=image> - Most of the form-related attributes on inputs including: form, formaction, formtarget, formmethod and others.

(Relaxing some of these rules might be reconsidered in the future - please let us know if you require these and provide use cases).

Usecase

if i have inputs on my form on desktop and use amp-sidebar's toolbar to put it elsewhere on the page at different breakpoints,
ill need the form to consider these as it's own inputs at all breakpoints.

(in my site , i am doing some massive filters and they are under one form on desktop and split across 3 sidebars on mobile , they use the toolbar feature to make the inputs and all functioning mechanisms non-redundant)

I think enabling the form attribute is not going to cause any content shifts (since developers normally dont target it in their CSS)