After an investigation, it was found out, that the charts were upgraded to Helm v3 a long time ago but then were downgraded back to Helm v2, because Rancher couldn't handle it & bugged out hard.
However, this still does not make full sense, since Helm v2 is officially not supported by Rancher since 2020 & yet, when those charts are installed anew in Rancher 2.7.1 & 2.7.4, then they land in the correct Apps section, where all new charts land in. Why don't they land in the Legacy Apps section?
So much in the Rancher UI just does not make much sense.
I'm now onto trying out different ways of making this work, including the too many times mentioned Longhorn conversion script, which is way less helpful compared to how many times it was already mentioned (& that wasn't even that many times).