The command doesn't work sometimes

Put the summary of the bug you're having here

What I expected to happen was the command will work
Describe what you thought should happen here

What actually happened was it said Unknown command
Describe what happened here sometimes when I put /setblock ~ ~ ~1 tnt it says Unknown command (It only happens sometimes

Steps to Reproduce:
1. give yourself a repeating command block
2. type /setblock ~ ~ ~1 diamond_block
3. turn it on
4. there is a chance it will say unknown command