Hi @Michal Ziemba ,
The following is the result of my test with the json you provided, everything works fine:
Do you have "append changes to existing text" turned on?
Since we can't reproduce your situation, we need to narrow down the problem and try some things.
1.Whether all SharePoint lists have the same problem? You could create a new SharePoint list to check.
2.Check if the same issue occurs with other SharePoint site collections.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.