Typing commands gets halted or truncated

Hi. I'm running 1.12.2 Java version on MacOS.

For some reason typing in "/" commands or even just chatting, my text can get deleted, or truncated or just refuse to enter more text.

Examples:

1. If I type "//set mine" - it won't let me type "c" to complete the string. Then minutes later it will.

2. I could be intending to type "Hi there" but after I type "Hi t", that dissappears and I'm left with just "here".

Is this a known thing? I've had it with multiple installs of MC across multiple installs of MacOS but I'm starting to play with WorldEdit more and being able to reliably type commands would be pretty sweet

Thanks!