I would assume the issue is that you are reading in (for example) 10 data points that match up with 10 date times. You then are writing the 10 analyzed points to thingspeak but you only have one date time that it matches up with.
If you want to store the date times of the incoming points, try something like this:
[data,timestamps] = thingSpeakRead(ChannelID,'ReadKey',readAPIKey,'Fields',4,'DateRange',[datetime(2019,7,6,14,00,00),datetime('now')]);
To fix the problem I would suggest to change the datetime after the 'TimeStamps' to something that reflects the appropriate timestamps of the thingspeak data. So, I would utilize the timestamp variable created above. Try this,
lastTime = length(timestamps);