For months I have been chasing an issue on my site where the internal index was dropping content nodes from the index and I could not rebuild it. It was confirmed by Umbraco Support that this was a known bug. They told me till a fix is released that I should just rebuild the index. The problem is that if I hit the "Rebuild index" I just get the message "The process is taking longer than expected, check the Umbraco log to see if there have been any errors during this operation". If I set he log level to "debug" no errors ever show up in the log but the internal index never gets populated.
I have posted on GitHub and was told to update the Examine version, which did not fix it, then they told me to contact Umbraco Support who (despite this only happing in my cloud hosted environment with them) told me this is not something they can support, even though they did also at one point tell me it was a known bug.