When a memo field is being truncated like that, usually there is some intermediate at work. A wizard, for example, frequently truncates long text when importing or exporting. Spreadsheets impose limits. Some functions use byte offsets internally so get truncated. Also, if there is a variant present and it thinks you have text, it might decide that you have "ordinary" text rather than "memo-sized" text.
I've seen this before but it is usually tricky to find the culprit. It has always been some form of intermediate that silently truncates for you.