Hi, thank you for your response. In fact, have not tried to physically compare the values. But because Sharepoint "thinks" it has been changed, it is very possible that Infopath form alters the value everytime I save the form. If the multiline field is without formatting, no changes are triggered.
I had to use another workaround. Created another field, where I store unformatted value of the same, what I have in the rich text field. On this unformatted value I am am hooking the flow. And the rich text value I am using to show to the users. It works.
But that problem is there still. I somebody is trying to go through the history of changes, he can see every time, that somebody changed the rich text comment. But nobody did. Not very nice...If this will be fixed sometimes (probably not any more), I will just remove the extra column and hook it back.