I finally had some feedback from Flespi who pointed that their broker has a limit of 16 topic per subscription request : flespi MQTT broker - MQTT 5.0 compliant, secure, fast, and free
Indeed I have 18 topics in my subscription request.
It doesn't explain why the request works when I restart Node-red but not on subsequent reconnections.
Unfortunately I don't see, as of today, any ways in the MQTT broker node to limit the number of topics per subscription request.
I will let the test with no Clean Session run for a couple more days and then split my access to Flespi across 2 different MQTT Broker nodes as a work around.
At the same I will post a feature suggestion to add a limit to the number of topics per subscriptions requests (done at Add option to limit number of topics per subscription requests)
