Same here regarding the star metal - got it in every campaign, sooner or later.
This afternoon I studied the random encounter code for an unrelated reason (it's in world_maps_h.nss), which is why I could pinpoint the reason will little trouble: some patch must have changed the trigger chance for this encounter from 2% to 35%. That's row id 7 (SP_SUPERMETAL) in "rand_plains". Axameter is row id 9 (AXE_IN_STUMP) in "rand_forest", and it's still at 1%.
The code goes through the list of encounters from top to bottom, trying to find one that is eligible (conditions met, if any, and not happened yet or repeatable). If it does find one then it does a random roll against the trigger chance for that encounter. For Axameter this means that the encounter will not get picked 99 times out of 100, assuming the code takes this encounter into consideration at all.
Basically, you have to travel in the right region and exhaust all eligible 'plotty' encounters that are listed before Axameter in order to have any chance (all plot-based encounters before Axameter have 100% trigger chance, except for one with 50%).
The terrain type code that chooses the "rand_forest" table is carried by the relevant event, EVENT_TYPE_BEGIN_TRAVEL, and this seems to be generated by the engine rather than by script code. That makes it difficult to check whether the terrain type depends on the point of departure or the destination. Travelling within the the Brecilian Forest region should cover all bases.
Last but least, the game has an internal trip counter, which it uses to suppress random encounters for trips #1, #6, #9, #12, #15, #19 and #21. Then the counter wraps back to 1.