Bonjour Imogen,

Pronominal verbs (se donner or s’inquiéter) should in theory agree with the subject of the sentence, especially as they are conjugated with the auxiliary ‘être’. And yet,… The reflexive pronoun represents either a direct object or an indirect object. And the verb agreement only happens of the reflexive pronoun is a direct object.

1. On s’était donné rendez-vous

There is no agreement because the direct object is ‘rendez-vous’ and not ‘s’.

donné quoi → rendez-vous

 

2. On s’est inquiétés

There is agreement because the direct object is ‘s’ ( ‘on’→ Fabienne et Pascal / plural).

Qui est inquiété → s’ = on (Fabienne et Pascal / plural)

 

Here is another link to a previous answer on a similar question.

 

I hope this is helpful.

Bonne journée!