Same here. I thought it was maybe only strike and public event bosses that counted because I did a public event and killed the frozen pod escapee on the Tangled Shore at Thieves Landing and that counted, then did two lost sectors, solo, and neither one did, so I'm thinking "forget lost sectors for this because they don't work". So I'm cruising around the EDZ and run into one of those giant purple-pool-ball public events and kill that thing using nothing but stasis to damage it the entire time (died like four times getting close enough to use Salvation's Grip on it) but it didn't count. It seems very wonky or at the very least, unforgiving, in determining what counts and what doesn't.