Reproduces in 6.9.1 (19.2) and above for lines at the bottom of the text box
Reproduces in 7.0.1 (19.3) and above for characters at the right of the text box
Exacerbated by 2020.1 change that allows you to zoom in past the default level, making it more difficult to find a zoom level where things display correctly. For this reason it might be appropriate to fix it for 2020.1 and not backport it, as the problem is more severe in 2020.1 despite existing in previous versions.
Repro steps (bottom):

1. Create a custom function node in a shadergraph, put it in "string" mode
2. Copy the test code from the first comment in this case (starting the with row with all the periods/fullstops, ending with the 2 at the end of the comment)
3. Zoom two levels out from default with scroll wheel down (to return to default zoom level, save, close, and reopen the graph)
4. Try to scroll to the bottom of the text box using the arrow keys or the cursor (to get to the 2 at the bottom)

Expected result: you can read any part of the text box you've moved the cursor to at any zoom level
Actual result: at certain zoom levels, you can't read certain parts of the text box even if your cursor is there

Repro steps (right):

1. Create a custom function node in a shadergraph, put it in "string" mode
2. Copy the test code from the first comment in this case (starting the with row with all the periods/fullstops, ending with the 2 at the end of the comment)
3. Zoom one level out (or in, in 2020.1) from default with scroll wheel down (to return to default zoom level, save, close, and reopen the graph)
4. Try to scroll to the right of the text box using the arrow keys or the cursor (to get to the 2 at the end of the periods/fullstops)

Expected result: you can read any part of the text box you've moved the cursor to at any zoom level
Actual result: at certain zoom levels, you can't read certain parts of the text box even if your cursor is there

Note: although I gave specific zoom levels this happens at, there are more zoom levels than what I gave the repro steps for.

Note 2: this problem has occurred with almost all complex code I have tried to write in the custom function node. It is not restricted to the test text I made.