🦸‍♂️ Not the solution jinja2 deserves, but the workaround it needs right now.

jinja2_workarounds offers an extension for jinja2 that works around a long standing issue¹ where include does not preserve correct indentation for multi-line includes. Simply add the jinja2_workarounds.MultiLineInclude extension to your environment and use the indent content directive to correctly indent your multi-line includes.

Installation

pip install jinja2_workarounds

Usage example

# text.j2
this
is 
some 
text

# example.j2
example:
    {% include 'text.j2' indent content %}

is then rendered as

example:
    this
    is 
    some 
    text

compared to jinja2's default include which would result in

example:
    this
is 
some 
text

Advanced features

MultiLineInclude is compatible with custom block_start_string and block_end_string. It also works with the advanced features of jinja2's include statement. The following variants are all supported and work as expected

{% include 'missing.j2' ignore missing indent content %}  # handle missing templates
{% include ['foo.j2', 'bar.j2'] indent content %}  # multiple alternative templates
{% include 'child.j2' without context %}  # include child with/without content
{%- include 'child.j2' +%}  # include with custom whitespace control

Footnotes

1. https://github.com/pallets/jinja/issues/178 ↩