Hey Tomas,
We're using v1.1.0 of pglogical. I can't honestly say what could reproduce this issue as I'm not entirely sure and haven't tested it yet.
I had thought it was caused when a toasted row is deleted before the pglogical_output plugin processes the event (so it fails when going to de-toast the row), but I would imagine that postgres internally should prevent deleting data that would be needed by a replication stream. I must admit I'm not well versed in this area, though. My understanding is wal2json doesn't detoast the data at all, and thus doesn't have an issue (for our case, we really just need the json data with an identifier telling us that the row changed, we're not doing full logical replication).
Best,
Colin