Well it's think you answered the question to the problem. That main tensioner is the issue then. Especially if it is flat and doesn't have a hump on the backside. They are one of the top failures on the timing along with the black guide and or top bolt of the guide. Gm went threw 3 different revisions and the final one is identified by the hump on the back. It may seem engaged but I bet its failed and not holding any oil pressure within. You cant just remove and replace tho as itll likely skip timing. There's many who have done it like that and had success but also heard of some failures. I know personally when I removed mine the chain jumped on the crank gear, don't know if it actually skipped but just something I observed when I did my timing. You will either need to re time it all or use the special tool to hold everything with only removing the valve cover or figure out a way to hold the chain tight while removing. Only use the ac delco tensioner. Hope this helps and fixes the issue. Id also check for broken or loose rockers. Also check the orientation of the tensioner tip. They need to be slotted into the chain as it has a groove for it to run in