Big drop in AHI

After 18 months I switched from a full face mask to a nasal mask. My AHI generally was under 5 with the full face mask. When I switched to the nasal mask it dropped to less than 1. Could there be an issue with the nasal mask? Air seal is 100% on both. I do notice air coming out of the front bottom of the nasal mask and a little coming from where the hose connects to the elbow. Is the nasal mask that much better at reducing events or is there a mask issue?

Don't see why one mask would yeild lower AHI than another if the leak rates on both are low. Only thing I can think of is if your machine has a setting for MASK TYPE. Mine does; has three and you have to switch when you switch types or the results will be skewed.

Actually, there is a documented effect where swapping from a nasal mask to an full face can cause an increase in AHI for SOME people. This is referred to in sleep tech slang as the "reverse MAS effect" as it is believed that what is happening is the opposite effect of using a dental mouth splint. The full face mask presses on the lower jaw which in turn puts more pressure on the air way making it collapse more and emphasising the obstruction. This can usually be countered with a higher pressure. I have seen it myself several times. With this in mind, it is not that surprising that swapping from a full face mask to a nasal has lead to a decrease in AHI. Lucky you!