I think there is a backwards compatibility issue with the data structures the source-only snapshot keeps on disk between snapshots here and an outdated version of this data is not getting cleaned up.
I'm trying to reproduce this currently and will open an issue for it once I have. I think you should be able to work around this problem by deleting the _snapshot directories in your shard folders like e.g. /mnt/istore/elasticsearch/data/nodes/0/indices/vDAnJnfiTkWOZ7nOv4GMDw/126/_snapshot so that the on-disk data structures are re-created with a supported Lucene version.
EDIT: I opened https://github.com/elastic/elasticsearch/issues/62700 to track a fix for this.