Question for Form experts: is this limitation just to allow form_rest() and the closing form tag to know which fields have been already rendered ... or is this limitation needed for some other strong reasons? Thanks!

There is the limitation of ids that are supposed to stay unique in the page, this could lead to labels not being correctly linked to their inputs.

This is specifically very useful to split form visually to two

By adding another submit button with different name and same label shouldn't be enough to cover the use case?

Yes, that's what I ultimately did. Not a fan of duplications tho.

A workaround for this is to set a var:

{% set saveBtn = form_row(form.save) %}

and rendering it twice (or as much as you need).

If you want to render the same form multiple times, you can use named forms for this afaik. Your screenshot is too small to clearly see how or what.

A workaround for this is to set a var:

You'll duplicate the id everytime you re-use the form field.

Symfony best practice: Add buttons in the templates, not in the form classes or the controllers.

Ok plenty things have been answered and even some good workarounds given. I am going to ask different question now. What's the reason for returning empty string for fields which were already rendered, if user tries to render them multiple times, instead of throwing exception with explanation? How about deprecating this silent behaviour?

@ostrolucky: It would make sense to me, indeed.

See #26531

If we disallow calling this for same field twice, we are opening up opportunity to change this behaviour in future.

E.g. in Symfony 6 we can make it no longer throw exception and allow to render same field more times, while shifting responsibility of making sure IDs do not collide to user, or even generate different IDs for subsequent fields. Currently this can't be changed due to BC.