I was having weird problems too i swapped the antennas and crazy as it sounds it worked for me too. Habitat really needs to sell placements!
I would put in a warranty request and see what happenes
The problem is that Z-wave operates in at 908.42 MHz in North America, and 868.42 MHz in Europe. Zigbee operates in the same 2.4 GHz band as WiFi. Technically, Zigbee is allowed to operate in the same bands as Z-wave, but that normally does not happen.
If you divide the 2.4 GHz frequency of Zigbee by the app. 900 MHz frequency of Z-wave, you get a ratio of approximately 2.7. While an antenna can be resonant at multiple frequencies, those frequencies must be harmonically related to the other band. This means the ratios must be 2, 3, 4, 5, 6, etc. Although 2.7 is "close to" 3, it is not close enough. It is close enough that you might be able to receive both band signals, it won't be close enough to efficiently transmit on both bands. That is why two separate antennas are required. Otherwise the radio would have to have a built-in tuner to compensate for a different SWR (standing wave ratio). Although use of an antenna tuner will match the transmitted signal to the mismatched antenna, there will be a loss of efficiency. The Hubitat could use the same antenna for both bands, but only by including an SWR compensating tuner circuit for one of the two bands. It is more efficient to have two antennas, each designed to operate at a specific band.
AFAIK both supplied antennas are identical. I have no idea how swapping them would have fixed the hub crashing, makes no logical sense.
Anyway... since you seem to know a lot about antennas, if you were to find some suitable replacements online and post about it I am sure a bunch of us could test them out and see how they work! I have been skeptical of the cellular style that work from 900Mhz-2.4ghz, but thats what other people have used.
If the antennas are identical, then there is a design option. An antenna designed to be resonant at 300 MHz would be resonant a 900 MHz at the 3rd harmonic. It would also be resonant at 2.4 MHz at the 8th harmonic.
The calculation for a resonant antenna is 468/frequency in MHz= length in feet
Thus for a 300 MHz antenna, the length would be:
468 /300 = 1.56 ft or 18.72"
For a quarter wave vertical antenna, the length would be 18.72" / 4 = 4.68"
The antennas are only 3" long. However, it is possible to use a shorter length if an appropriate amount of capacitance or inductance is added to the circuit. That is what antenna tuners do. They use added capacitance and inductance to "tune" the antenna.
They are... 
