Currently, the default Roblox chat will occasionally not send messages, even when the enter key is pressed and the message doesn’t have any filtered words. The bug happens about every 5-10 messages.
I have been able to reproduce this bug by just chatting text in any game that uses the default Roblox chat. As you can see in the video below, the bug only occurs occasionally and will only last a few tries before it allows you to chat again. Note that I haven’t set off the spam filter.
I unfortunately have not been able to test this on android device but I have been able to reproduce it on an iPhone and iPad. Both devices had the latest version of Roblox installed (2.385.303034) and the latest version of iOS (12.3).