When I have a textblock Lectora 17.1.3 with “Vertical Scroll” checked, it will have overflow-x:hidden and overflow-y:auto. It seems to work alright with Change Contents actions both directions, i.e. changing from text with scrollbar to shorter text and vice versa.
The earliest moment to change the content would be to either use an “On: Page show” action at the textfield or to use inline variable replacement by prepopulating your field with “VAR(your_variable)”
I tried both OnPageShow and the variable technique. Neither updates the contents of the field until you navigate. However navigation back to the page you are on works and is extremely fast, so I can use that without a problem. It happens so fast that page audio doesn’t even start before the page reloads. There isn’t even a visible flicker.