See pic and Layout file. Size A0 paper. Using default text in layout. When rotating the text 90 degrees, some parts of the first letter are cut - it depends on the position on the paper.
I can reproduce it. This appears to be related to the distance from the page’s origin (top left corner) to the text. On large sheets text on the far right and/or bottom get this cropping worst.
We have seen this before, but we haven’t had time to determine the root cause of this issue. As @DaveR said, it gets worse the further from the page origin you get.
On a related note, the position of the text shifts as the text box is rotated.
In the image below the text box highlighted in blue is nicely centered both vertically and horizontally. Doing an array for example illustrates the glitch.
Digging a little more, it seems related to the OP in that the closer you are to the upper left hand origin corner, the less noticeable the problem is, the further you go to the lower right hand corner the more extreme.