It can happen sometimes that in some dgn files that have as Original file format V7 or DWG the text editor ruler is not the width of the dialog box and, in some cases, it is not even the length of the text string. If it happens, it happens only when editing Text and not when inserting new text.
1.Grab one of the corner angle handles and drag for rezising the dialog.
2.Double click on the text and you will see that the ruler is now longer.
Delete the affected text string and re-insert it from scratch.
How to increase the allowable line length of text in Word Processor