Something weird is happening with this keyword?!

[CongthanhThe]

Hey guys,
In the past weeks, i was ranking #4 for a keyword and as a result, I was getting a lot of traffic and I made some unremarkable modifications to page content and reindexed it.
after that, i noticed a drop in traffic, and when i checked it out, i noticed that the page is not ranking for that keyword anymore, it's no longer in the search engine (not even in page 15).
The funny thing is that if i add any letter to the main keyword (for example "Keyword" c) and search for it, my page ranks #1.
It's as if google bots are avoiding to rank my page for that main keyword only.
Ps:

• I didn't make any black hat SEO for that page or my website in general.
• No issues with other keywords
• i reindexed the page several times and what i noticed, is that in the first 4 hours i restore my ranking whit that keyword but after a while, the same problem occurs (My site disappears from the search engine)
  If you have any idea about this issue, i will be grateful if you could help.
  Thanks in advance