Send an email to the mods hn@ycombinator.com , they usually reply soon.
I don't think this is shadowbanning. I have another definition of shadowbanning.
A post with a few flags get a small penalty, and with more flags it gets a [flagged] label and more penalty, and with even more flags it is autokilled. Perhaps the post got only a few flags? Perhaps you are overestimating the normal decay?
About the Tulsa riot, this site has a lot of users from USA (60%?), so it has a USA bias. [Hi from Argentina!] Post related to other countries sometimes get ignored.
Also, there is a bias for current events, and things related to current events. If tomorrow Turkey decides to bomb Athens, my guess is a repost would get more traction. (But I hope we can never test my hypothesis.)