The native high dev is not a bug and is WAD.
This have confused a lot of people. Unsettled tribes can only control one province, but they can 'own' multiple provinces (you can see this from the unshaded provinces with color border). Unsettled tribes have to regularly migrate between different provinces in their domain due to devastation mechanics.
The reason the native OPMs have high dev is because they will have to use their dev to eventually settle the other empty provinces in their domain. Settling another province will reduce their dev.
1.31.1 just fixed the bug where you can pillage native capital without lowering their dev, which can be kinda OP.
See the dev diary:
Hey everyone! So there’s been quite a few development diaries from @neondt now on South East Asia telling you about the new content we’ll be adding to it, a region that had gotten a bit missed by us over the years and which very much needed some...
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