The problem: in eZP, remote_ids are string fields, which can contain any character. They are often used to link contents/locations to remote systems, and as such it is quite common for developers to fill them in using syntax similar to: 'external_10', 'external:102' or 'external/1023' etc...
In eZFind, those remote ids work as is: they can be searched for and used in filters.

The searchbundle code takes a surprising approach, and strips any non-alphabetic char when sending the remote-ids to the search engine.
This makes it impossible to later find them.
In other words: this breaks the Search Criteria which deal with remote ids.

The culprit: https://github.com/ezsystems/ezplatform-solr-search-engine/blob/769c6be78d9f18381ba8afd82b03d5e77e680c5c/lib/FieldValueMapper/IdentifierMapper.php#L56