chat_postMessage inline code block with >4000 symbols broken markdown

I'm sending large inline code blocks into our slack rooms and blocks with more than 4000 symbols (roughly) are coming with a broken code style.

This message works fine:

"*test*\n```#{"x"*4000}```\n*test*"

This one came "broken":

"*test*\n```#{"x"*4100}```\n*test*"

It seems that the message is being split into two.
I'm not quite sure whether it's a gem's issue or an API problem.

The doc https://api.slack.com/methods/chat.postMessage#formatting says

For best results, limit the number of characters in the text field to 4,000 characters. Ideally, messages should be short and human-readable. Slack will truncate messages containing more than 40,000 characters.

Messages are not truncated though, just split.
Please guide where to look at.
I'm using v0.14.1 at the moment.

Start at the chat_postMessage entry point and reproduce the issue using the two payloads in the report with 4,000 and 4,100 characters. Compare the client request and Slack response to determine whether splitting happens in slack-ruby-client or the Slack API; done means the cause and expected handling are established.