Thank you for your tips, at least I now have more knowledge about onewire.
Last night there was something else in between, but now I have put the sensors in a daisy chain (see photo).
First I tested it with the "standard" Arduino dallas temperature sketch "oneWireSearch", I got up to 7 sensors in this one.
I connected the cable of the waterproof sensors to the three wires (GND, 5V and the onewire pin (pin7)), threaded and twisted together.
When I connect an 8th sensor, the search sketch says that no sensors can be found, it does not matter which sensor is the 8th.
If I connect the 7 sensors to the gateway node, I see the 7 sensors in domoticz, so that's where it is.
it is then 6 wired sensors and a sensor on the PCB.
does anyone know if the onewire bus might be a twisted pair?
or is the distance between the branches too short?
If the problem remains, I just make extra oneWire bus for the 4 ground temperature sensors as you already suggested ...