I asked for the belongsTo just to know if that works as expected (If you are viewing the firstname in it instead of an id).
The Référence part in the URL seems weird to me (accent should be encoded, but that seems to be an issue from our side), but I still did a quick test on my end (Renaming my field to Référence) with no issue related to that point. I would still suggest to try renaming it to “Reference” just to see if that helps.
Also, fields[Fundings] does not looks right to me. I’ll dig into this, but I’m pretty sure collection name should be generated in lower case. Did you change anything related to that part ?