estimatesmartfee also returns blocks, which the user would expect to be the estimated number of blocks for the confirmation, but that's not the case, because often when you query an estimation one target under the returned blocks number, the feerate won't differ.
I think the returned blocks depends on how long your node has been running. So in the first case above it's really returning the maximum number of blocks for which your node has evidence of the appropriate fee rate. I don't think this is a bug.
the maximum number of blocks for which your node has evidence of the appropriate fee rate
I'm not sure I understand. Could you explain it in another way?
Sorry, I just mean that, at the time you ran the command, your node didn't have more than 38 blocks worth of recent data with which to estimate fees. So when you pass a value larger than 38, it just gives you the estimate for 38 blocks because that's the best it can do at that time.
From the command help:
The request target will be clamped between 2 and the highest target fee estimation is able to return based on how long it has been running.
I believe the fact that 37 gave you the same fee rate as 38 is just because the estimates are based on fee rate buckets, and it's totally possible for 37 and 38 to happen to result in the same bucket.
I see. That explains a lot. I misunderstood what the blocks return value mean. Then it's not a bug. Thank you!