Text box is truncating its content if placed inside the rectangle.
A very common issues occur when you placed a text box inside a rectangle container. Inside a single rectangle container, you can put multiple text boxes, but issues appear when you wanted to increase the size of text box dynamically. Microsoft has provided an update for this issue, which you can find at below link. This update will fix the problem of dynamic size of text box.
https://support.microsoft.com/en-in/help/4338040/rectangle-does-not-grow-together-with-textbox-in-ssrs-2016