@CCS86 This limit has been defined because at a certain size the HTML textarea becomes too slow to work with. We might be able to remove that once the new G-code editor from the pending DWC PR has been merged.
It can be reproduced even with newest 2.1.8. Load the page, force reload (ctrl+f5), type any command in the input field and press the button. Nothing happens. Press again and it sends, and seems to work untill a new force reload.
In the source there is a comment "// FIXME There must be a better solution than this", so I guess chrishamm is aware.