Okay, I think I understand now. I assume the field in both lists is multiple lines of text. If so, are they both configured the same in both lists. For example, are they both set to plain text or rich text:

If you are using rich text, do you have append changes set to yes. If so, that is likely your problem. Power Automate doesn't support that.
I have only built flows with lists that use plain text. If you are using rich text with append changes set to no, that may be your problem, but I could not be certain since I don't set things up that way. I did ask Copilot, and it says you need to use an HTTP request to copy rich text fields. I am pretty sure I use plain text all the time because of the problem you are having. But I have been doing it this way for so long I may be wrong and maybe it is now just habit.
If you have access to Copilot, this is what I asked: When copying a SharePoint list item using Power Automate, if I have a field that is multiple lines of text and rich text is enabled, are there any issues with this in Power Automate? I am not sure its response was accurate, which is why I didn't post it. If you don't have access to Copilot, I can ask again and post it.