Rest API tax_relation=OR doesn't seem to work correctly

[roverlap]

Let's say we have posts of a custom post type.

Post 1 has custom-tax1 with ID1 and custom-tax2 with ID2.
Post 2 has custom-tax2 with ID2.
Post 3 has custom-tax1 with ID3.
Post 4 has no custom taxonomies attached.

When querying

/wp/v2/custompost?&_fields=id,title&custom-tax1=ID&custom-tax2=ID2

it returns only Post 1 as expected.

/wp/v2/custompost?&_fields=id,title&custom-tax2=ID2

returns Post 1 and Post 2 as expected

Querying

/wp/v2/custompost?&_fields=id,title&custom-tax1=ID1&custom-tax2=ID2&tax_relation=OR

I expect it to return Post 1 and Post 2 as well, but instead it returns all 4 Posts. As soon as tax_relation=OR is added to the query, it seems all prior tax queries are just ignored.

Unless I'm misreading this ​https://make.wordpress.org/core/2020/02/29/rest-api-changes-in-5-4/ and it should somehow work differently.