So you have to identify where is the cause of your issue, right?
There are two options here:
- flespi somehow sends messages with only first 10 keys
- your server software somehow reduces each received message up to 10 keys
But you can easily test option 2 by sending a large message with 15 keys (for example) using cURL tool just like this:
curl -v -X POST --header 'x-api-key: X' -d '[{"k1":1,"k2":2,"k3":3,"k4":4,"k5":5,"k6":6,"k7":7,"k8":8,"k9":9,"k10":10,"k11":11,"k12":12,"k13":13,"k14":14,"k15":15}]' 'https://your.server.url.like.in.stream'
If you will debug print received messages from that cURL request and still see only 10 keys - so issue is caused by your software.
This is very simple but important test in your case.