I think I might be able to answer why the 0.8% estimation at least may not be correct, then. The whole “table 1, 2, 3” thing is a misnomer based on the way they were stated to do the pulls. Sirrian specifically states here that they pull from the last table first when determining rewards. LTs have gone through a couple reward shifts since then with the runics being added and now with gems and minors being removed, this should still hold true. Therefore, a mythic at 0.5% on the last table would equal a 0.5% chance to get a mythic in the task, period. Your sample would then still be an outlier, but not nearly as big of one.
If you have some data on what the other tables might look like, specifically gem keys, event keys, glory keys and glory, I’d love to see them. I am particularly interested in what ~3.9 LTs will, on average, get us in mythic hunting resources, as this equates to the amount of gold we would have to put in to LTs after finishing Guild Tasks each week to equate to 25% more gold being put into tasks total (the amount of total more gold reportedly entering the economy that the 20% Guild Task reduction was allegedly done to compensate for). If you happen to have the old LT values saved somewhere for comparison purposes, that would be helpful as well.