What I’ve seen happen when timing belts/chains with hydraulic tensioners are installed, and if there’s any slack anywhere else in the belt/chain other than at the tensioner, the engine can be rotated by hand and the marks stay lined up.
But when you start the engine, and the oil pressure forces the hydraulic tensioner against the guide, any slack will be removed out of belt/chain. and the only way this can occur is by causing the cam shaft sprocket(s) to rotate slightly, which then causes the valve timing off.
And when that happens, bad things can occur.
Tester