Here are some alternate explanations of the data, all hypotheticals that should be considered before OPs conclusion is accepted as an accurate interpretation of his data:
Practitioners of different languages have different reporting habits: they call different things "bugs", they report with varying frequencies, they tend to not care about reporting bugs as much as building the next feature, etc.
Bugs are different sizes, so while haskell and python might both have "1 bug", the cost of that bug could vary wildly.
There's a ratio of "bugs per feature", so more productive languages show up as more buggy.
Bugs are labeled differently, IE perhaps haskell projects tend to have nice "bug" labels just because static typists are more OCD about it, where as a python project might have a million bugs, but no one labeled them as such. (related to my bullet #1)
I agree with others in the comments, in order to appropriately draw up causal relationships, one would need to construct an appropriate experiment. Double-blind-placebo-controlled-randomized might be a bit tough to construct, although the closer to that one could be, the better.
Perhaps one could construct a randomized crossover though, and that would finally lend some actionable insights into the problem?
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