One of the more common failures that we see in designs occurs when a structure will fail into the shorter span. The root cause is likely one of two things: the 'Average Length on Main Span' setting, or the temperature change in your load case.
If a client file is set up to use different tensions for the two spans (not using a single ruling span), then the 'Average Length on Main Span' option will average the two tensions and initialize both sets of wires at this tension. If the difference in span length is extreme (for example, 400' in one direction and 150' in the other, both with vastly different tensions), then the average tension will be extremely low for the long span, and extremely high for the short span.
If you are applying a single tension to both spans, then the likely cause is the temperature change in the load case. When a single tension value is applied over a short and a long span, the short span will always have less sag in the line. Because it has less slack, when the wire contracts due to the colder load case temperature, the short span increases in tension faster. In some cases, if the ruling span tension is high enough, or if the short span is short enough, there is virtually no sag in the wire, causing it to jump to a high-tension state when it contracts.
In reality, these poles would likely deflect a few centimeters into the shorter span and quickly equalize the tension imbalance. This relatively small deflection is all it would take to slightly add slack to the short span and balance the tensions. However, accounting for this tension balancing is not possible with the linear analysis method most users are accustomed to.
Linear analysis is a single-step process. It simply calculates the initial forces (specified by the load case) acting on the pole and the stresses they would cause. It does not account for changes in these forces due to the pole shifting or deflecting.
However, accounting for these changes in tension is possible with co-rotational analysis, which is an iterative method. Enabling the load case option 'Update Tensions Based on Displacement' allows the tension for each wire to be updated as the structure deforms and deflects. With this option enabled, the structure will start to deflect into the higher tension span until the tensions begin to balance, with the higher tensions being reduced and the lower tensions increasing.