On December 13-14, 2018, we've remove spam from your channel and hence you see a noticeable decrease in your subscriber count. More info here: https://goo.gl/pGubnn . Hope this clarifies.
Appreciate the info. Please send a feedback report directly from YouTube with these steps: https://goo.gl/WdI0BK . While every individual report may not get a direct response, we do review everything and use the details to investigate and fix issues. Thanks!