arbitrary UNSAFE_TRANSACTION errors when using sql.begin

The following query arbitrary causes an UNSAVE_TRANSACTION error.

await this.sql.begin(async (tsql) => {
  await tsql`DELETE
            FROM mapping_table
            WHERE a_id = ${aId}`;
  if (items.length === 0) {
    return;
  }
  await tsql`INSERT INTO mapping_table ${
    // eslint-disable-next-line @ts-safeql/check-sql
    tsql(items, 'a_id', 'b_id')
  }`;
});

the error most likely happens when this code is executed very frequently and items is usually an array betwenn 0 and a few thousand items.

error:

Error: UNSAFE_TRANSACTION: Only use sql.begin, sql.reserved or max: 1
    at CommandComplete (/Users/juhwon/Projects/xx/xx/node_modules/postgres/cjs/src/connection.js:580:29)
    at handle (/Users/juhwon/Projects/xx/xx/node_modules/postgres/cjs/src/connection.js:476:6)
    at Socket.data (/Users/juhwon/Projects/xx/xx/node_modules/postgres/cjs/src/connection.js:315:9)
    at Socket.emit (node:events:514:28)
    at addChunk (node:internal/streams/readable:376:12)
    at readableAddChunk (node:internal/streams/readable:349:9)
    at Socket.Readable.push (node:internal/streams/readable:286:10)
    at TCP.onStreamRead (node:internal/stream_base_commons:190:23)
    at TCP.callbackTrampoline (node:internal/async_hooks:130:17)

to my understanding using sql.begin for something like this should be fine. have we found a bug here or is there something wrong with our code?

Start with the sql.begin entry point and reproduce the supplied DELETE and conditional INSERT under frequent concurrent calls. Inspect the transaction handling around the UNSAFE_TRANSACTION guard and compare it with the connection.js stack trace. Done means establishing whether this is a client bug or caller misuse, with the behavior covered by a reliable regression test if it is a bug.