Around once in every 4 months, the YouTube algorithms clear out all subscribers, views, etc which are detected as bot generated as per the algorithms of YouTube. So, low quality bot generated views, subscribers will drop and if this happens often, then a channel can even get penalized.
One of the ways to avoid this and gain real views, subscribers, is to promote YouTube videos through video promotion campaigns of Google Ads, which is very much within YouTube's TOS. If the Google Ads video campaign is setup properly and with effective targeting, then it often leads to good amount of views and also real engagements (likes, comments, subscribers). Ofcourse the video needs to be appealing & with good content.