Hi,
Since I solved the issue of swapping memory on my hypervisor host (impact on overall system performance of the virtual machines) the issue is gone.
More than 36h running zwavejs2mqtt with no issue and no need to restart anythin! 
As I explained above, in my last comment, my HomeAssistant consume more RAM than before (between 1.5 and 2 times more) . And I had almost 1GB of RAM swapped to disk, maybe for that reason there was strange behaviour in interaction between HA and zwavejs2mqtt.
Maybe (I don’t say that’s the reason but just an analysis by myslef of what might occurs) something like : HA waiting ACK from zwavwjs2mqtt add-on, but as zwavejs2mqtt is possibly slowed down by memory swapping, it can,'t answered in a fashion time, but continue to treat correctly and to prioritize it’s own internal communication with its zwave devices => in reality no error reported in add-on, logs. On the other hand, HA in unable to dialog with it and reported that it is not available in its core-logs. Restarting add-on might flush addressed memory in swap and new memory will be loaded in “real” RAM instead in the swap. After a certain amount of time memory is progressively moved to swap, then the issue come back again.
No body… no murder
This is the same for the errors:
No error… no issue
, this is difficult to troubleshoot when no relevant error is reported.