I've never heard of this. If all the added notes pertaining to a record are in one field he only way I can think that this would happen is that somehow the cursor placement is wonky. Whether or not this can happen if the method of adding notes gets switched from editing directly in a table to being done via form, I have no clue. However, it may be an undiscovered reason to add to the list of many reasons why one should not edit directly in tables. Or maybe there's something about the code that we can't see, such as using Ribbon commands or functions to enter a form field and add the text, which might place the cursor in the wrong place.
I think your question is taking a turn toward figuring out a way to reconstruct my database rather than simply trying to figure out if Textbox behavior has changed.
Sometimes the message is one that the OP doesn't want to hear, I'm sure. In this case, I'll tell you anyway that the right method is to append your comments to a table of comments (as a new record each time) and add the edit date to the CmntDate field. This is a common approach, such as adding to a list of work order task completion comments because the WO task can be in progress for several days, being worked on by several people. Besides that, you will find it increasingly difficult if not impossible, to isolate comments between 2 dates, or all comments prior to or after a certain date, etc. The right way also doesn't require you to insert carriage returns or write even the simplest procedure to make the insertion.
I will get off of my soapbox now!
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.