Search queries don’t work and redirects to default language

I did a custom form with searchform.php in order to make more complex searches. Using pll_home_url() has been easy.

However, with anything more complex, e.g. a combination of searching text + categories, the search queries don’t work.

Default language: EN.
Secondary language: FR.

Example 1:
This URL does work:
[home_url]/category/dogs/?post_format=post-format-aside&s=
However, by using the language switcher it becomes this:
[home_url]/fr/category/chiens/?post_format=post-format-aside&s=
which redirects me to English.
Trying without the “fr”:
[home_url]/category/chiens/?post_format=post-format-aside&s=
didn’t work either: it still redirects me to English.

Example 2:
This URL does work:
[home_url]/category/dogs/?s=
But this redirects to English (expected the French page about dogs):
[home_url]/category/chiens/?s=

Example 3, working in the default language only (English):
[home_url]/category/dogs+cats
Redirects me to dogs only even though there are posts with both the dogs and cats categories.

Permalinks settings
“Post name” [home_url]/sample-post/
“Optional” section untouched

Using a static front page

Polylang settings
URL modifications:
“The language is set from content”
“Hide URL language information for default language: checked”
“Remove /language/ in pretty permalinks”
“The front page URL contains the language code instead of the page name or page id”
Everything synchronized.

What I’ve tried
Tons of different URLs; nothing works.