The bug
In the "Video Settings" menu, if you try to change your graphics setting to fabulous while using a computer that does not support the fabulous option, you will be shown a warning screen. If you click "Take me back" to exit the warning screen, then when you continue playing in your world translucent blocks will no longer render. Relogging, using F3 + T, or going through a nether portal will cause translucent blocks to render again. Completely transparent blocks such as regular glass render correctly. Also, while this bug is occurring, leaves will render as if the fancy option is selected even if the fast option is selected.
To reproduce
Please see the attached video.
[media]
Affected blocks (There might be more)
Water
Stained glass
Stained glass pane
Nether portal
Slime block
Honey block
This does not occur in 1.16 pre-release 5. I suspect that this might be related to a change that occurred in pre-release 6. In pre-release 5, switching between graphics settings would show the warning screen every time the player clicked on fancy (because fabulous is the option after fancy) and clicking "Take me back" would set the option to fast. In pre-release 6, the warning screen is shown once per visit to the "Video Settings" screen. After seeing the warning screen once during the current visit to the "Video Settings" screen, clicking on the graphics button will switch back and forth between fast and fancy. Additionally, despite the resolution of MC-188981, clicking "Take me back" in pre-release 6 will (always) set the option to fancy.