When I played on 19w35a the Wandering Trader was spawning always around 10 to 16 in game days. But now it doesn't for 30 in game days. I recognized it as a bug since Trader issue MC-153298 has been fixed. So maybe this also bugged spawning above the bedrock layer. I know that max  chance of Trader spawning is around 7%, but this is just strange.

World type: Buffet, Dark Oak Forest, Type: Caves

Platform is 41x41 1 level above the bedrock layer