I found this YouTube Help thread interesting because it talks about how they improved their spam filters around fake likes or dislikes. Specifically, starting yesterday, June 14, 2018, they pushed out an update that takes aim at blocking likes “purchased from a third-party service.”
This is aimed at paid likes, like paid links in the SEO world.
Lukas from YouTube wrote:
Starting today, we’re making a system update to catch even more spam in likes and dislikes across YouTube. The main reason a like would be marked as spam is when it’s purchased from a third-party service. Paying for likes or dislikes is a direct violation of YouTube’s Terms of Service.
If more likes or dislikes on your channel are marked as invalid than before, you may see fluctuations starting on June 14, 2018. Learn more about our policy and likes reporting here. You may also notice that the number of likes or dislikes in YouTube Analytics is different than what you see on the watch page under the video. This is a known issue and our team is working to fix it. In the meantime, view the counts on the watch page for the most accurate number.
So if you are big into YouTube and buying likes, you may have seen a drop in the number of likes on some of your videos.
Forum discussion at YouTube Help.