The bug

The /spreadplayers ... under option which was added in 20w21a does prevent invalid maxHeight values where it is never able to spread players, when parsing the arguments.
These are values <= 0.
Additionally it does not prevent values > 256 even though they are effectively the same as 256.

Reproduction steps

1. Enter the following in chat

   /spreadplayers ~ ~ 1 10 under -1 false @s
   

   The command is considered valid

2. Try to run the command
   It fails, as expected