Meta queries after first meta query stripped from nested meta queries

[christianc1]

Describe the bug
When you are nesting meta queries more than a single level deep, all but the first meta query is stripped out of the deeper queries.

Take for example:

$meta_query = array(
    array(
        array(
            'key' => 'key1',
            'value' => 'on',
        ),
        array(
            'key' => 'key2'
            'value' => 'off'
        ),
        'relation' => 'AND'
    ),
    array(
        'key' => 'key3',
        'value' => 'red',
    ),
    'relation' => 'OR',
);

In this example, the meta query on key2 get's stripped out because of https://github.com/10up/ElasticPress/blob/develop/includes/classes/Indexable.php#L488

Steps to Reproduce

1. Nest multiple meta queries at a depth of 2.
2. Inspect EP request
3. meta_query[0][1] is missing.

Expected behavior
Meta queries should not disappear.

Screenshots

Environment information

• Device: Macbook Pro
• OS: MacOS 10.14.6
• WordPress version: 5.4.2
• EP Version: 3.4.1

[christianc1]

Note, you can workaround this issue by ensuring that relation key is the first element in the array, but that's sort of luck. Happy to submit a patch, but the code causing this issue looks like it's in there to handle another strange edge case. Maybe we can discuss the original intent more @tlovett1.

[Jamiewarb]

Thank you @christianc1 this comment has saved me from more days of pulling my hair out trying to understand what's going wrong.

Would love to see this fixed if possible, or potentially documented as a known bug in the docs