You are correct, the E configuration is generally found to be the most stable isomer. It is therefore not suprising to see that in the final liquid state a higher E isomer content is found. The final ratio of interconverting E and Z isomers that you measure in the liquid is determined mainly by the energy difference between the two and the temperature.
However, in the solid state the E isomer does not necessarily give you the more stable crystal structure since the packing of the individual molecules can be very different for each of the two isomers.
I was involved in this research project and can add that interestingly the solid form of the E isomer of this chemical has not been documented so far. It appears that under the conditions that were used, the E isomer of this molecule is found only in the liquid state.
However, in the solid state the E isomer does not necessarily give you the more stable crystal structure since the packing of the individual molecules can be very different for each of the two isomers.
I was involved in this research project and can add that interestingly the solid form of the E isomer of this chemical has not been documented so far. It appears that under the conditions that were used, the E isomer of this molecule is found only in the liquid state.