I have reproduced ... 
So it looks like you are loading the template via the API and then going to the UI... and stepping through... I get the same error
The offending code appears to be this.
I would need to look what _data_stream_timestamp is.. I do not notice it in any of my OOTB templates not sure where you got it from...
"_data_stream_timestamp": {
"enabled": true
}
If I take it out and LOAD via the API it seems to work... if I leave it in I get the same error..
Also I went through the UI to try to create the mapping and tried to load the mappings via JSON and this is what I got
if I take out
"_data_stream_timestamp": {
"enabled": true
}
It loads Fine
