LCD Output via SoftwareSerial broken on IDE 1.8.10+ (1.8.9 OK)

I am not positive where this goes and am posting in arduino/Arduino requesting help in moving this to where it belongs.

The short description behavior we are seeing is that when using the SoftwareSerial library instead of the regular TX pin to write to an LCD, text does not seem to print correctly on and the cursor location seems to move around so that characters end up where they shouldn't be. This is new behavior after years of everything working fine with the same product and code (so only the IDE or possibly the library has changed).

The issue is described in detail in [this Arduino forum post](https://forum.arduino.cc/index.php?topic=667347.0) including all hardware, code, etc.

I have verified this issue still exists in the hourly build as of the time of posting. Therefore, the issue exists in 1.8.10, 1.8.11, 1.8.12 and the hourly build as of posting. Version 1.8.9 works as previously / expected. This is on Windows 10, using Arduino Unos, and [this LCD](https://docs.google.com/viewer?url=https%3A%2F%2Fwww.parallax.com%2Fsites%2Fdefault%2Ffiles%2Fdownloads%2F27979-Parallax-Serial-LCDs-Product-Guide-v3.1.pdf).

Start with the linked Arduino forum post, which contains the hardware and code details, then compare the reported behavior in IDE 1.8.9 and a newer build. Reproduce on the stated Windows 10 and Arduino Uno setup with the LCD; done means text prints correctly and the cursor stays in the expected location.

Written by the indexing model from the issue text.