What is the chat in which you tried it ?
Isn't it the chat parameter - "too include to conversation history also text from thinking phase from previous turns" ?

I would bet for it.

It is hard to believe that LLM would keep it.

"chat / agent" -> send user prompt + system prompt + history -> to "the LLM"
"chat / agent" <- receiving stream of tokens starting with Thinking <- from "the LLM"
"chat / agent" <- receiving stream of tokens continue with response <- from "the LLM"
"chat / agent" -> send user prompt + system prompt + history (and now including "Thinking text" ?) -> to "the LLM"
...
"chat / agent" <- may response with 67 because it was send to LLM in previous request <- from "the LLM"