Mica will not stick to glass unless it's capped with clear.
Almost by definition, any material that needs to be encased, does not stick to glass.
However, mica does stick to glass. But it is only the surface that is in contact with the glass that sticks. Mica shears into very fine sheets and particles (almost microscopic), meaning that there many layers of mica even with a thin layer. So only a minor portion of the mica you sprinkle, sift or paint onto the glass can stick.
It is possible to add a flux such as borax to the mica solution to soften the surface of the glass, allowing more mica to sink into and stick to the glass.
Of course you can encase much more mica than will stick to the surface. However, you have to be very careful about avoiding bubbles. There is so much air (relative to the volume of the mica) that bubbles in encased mica is a constant problem. Very good bubble squeezes and supporting the edges on shards of glass to keep the glass open while beginning to slump are required.
All myths have an element of truth in them otherwise they would not persist.
They also persist because people listen to the “rules” rather than thinking about the principles and applying them. It is when you understand the principles that you can successfully break the “rules”.