Assuming you're not drinking brown bottle beer when changing the oil, it's not glass, it's 'varnish', a by-product of gasoline as it chemically breaks down. Unburnt fuel does end up in the oil as do pieces of varnish that break free from interior surfaces of the engine - it's why oil contains detergents. They circulate to the filter and are captured. Perfectly normal.
You can confirm this by placing the pieces in a saucer filled with a solvent, even alcohol. Leave it overnight and you'll find that the 'glass' magically disappears.
Cheers!
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