I believe you have fulltext search activated. The MySQL database fulltext search has some wierd disadvantages of which one is a so called “known words removing” – basically the database simply discard some words if it's considered as a common word. I think “tube” might be considered as a common word and that's causing the problem.
The solution is very simple. On the Fulltext settings page just disable the Fulltext search option: https://i.imgur.com/1BcUDJ5.png
Sorry for the late answer, I've been working on a knowledgebase for the plugins and I simply forgot to answer support tickets.
Let me know if it helps!Best,
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