Hello Edwin,
thanks for your enhanced photos. It's interesting how our judgement can be subjective and our perception is probably based on previous experience. For me the photos just show/confirm the 'thingy' is in fact a feather, and the whole structure can be seen there with care, albeit in a low resolution.
The structure is not too long for a ventral feather at all, those feathers can be very long and cross the central belly from each side, concealing the incubation area (where the incubation patch is located during the breeding season).
I could not find ideal Mangrove Pitta photos to illustrate what I mean, but there are some of the similar Blue-winged Pitta:
1) one with a similarly naturally displaced feather (
HERE), but seen from below, so that the feather is seen in its full shape in a perfectly ventral angle; the OP photo shows the feather from a frontal angle, looking more 'blade-like'/much narrower.
2) expanding the lower belly feathers (
HERE)
3) spreading the lower belly feathers (
HERE), so that the skin below (where the incubation patch becomes developed, during the breeding season) becomes visible; these feathers are really quite long.
4) another individual showing displaced feathers (
HERE), but here the feathers show the normal orientation (inwards), instead of being twisted outwards as in the OP photo.