@Nicolas @Theo when you have time, could you give an update on what you think is the cause the communication problem? and what you have been testing? (here or another post, we are all (at least me), really curious about it.
Do you think the voltage not going down to zero, but rather 0.7, could be the source of the problem? The detection threshold should be above that.
If you moved to 200Ohm for the pull-up: the lower the pull-up resistor the more you have a voltage divider effect when pulled to zero with the wire resistance of other resistance along the way. A 30Ohm resistance on the way would make this 0.7 V difference, with 10 motors it might not be that impossible.
Have you tried measuring with 1, 2, 3, …15 motors to see the difference in the signal?
The post of @juju also raise some good questions: Informations sur la carte pixl (raspberry Pi - XL320)