Ionic Vue nested list - connected arrays

I still didn’t solve this but an update:
I am using a Map now. like this

const trackMap =  ref<Map<number, Track[]>>();

whereas the key number is the Record.id. the second ion-item line looks like this

<ion-item :key="track.id" v-for="track in trackMap[record.id]">

however this doesn’t work. the map is correctly filled but somehow there is no two-level nested display of the records with its tracks. but only the first level of the records. no error. no tracks. does someone has an idea?