This is condo constructed in 2006. When I moved in (2017) everything worked fine (unless I simply didn't notice?), lights were incandescent.
In 2019 I changed the master bathroom light fixture from Incandescent to a new LED one, and installed a can light in the shower, piggy backed from the wiring near the new fixture. This was done with help from an electrician friend, I am pretty confident that everything here was proper.
Later that year, I changed the common bathroom light fixture to the same LED fixture.
Last year my GF moved in, which was the first time I've had more than just myself at home. She told me that she has been noticing weird things with the the light in the common bathroom.
Both bathrooms have two simple switches, one for light and one for exhaust fan. The bathrooms have a shared wall, here is the layout: https://i.imgur.com/PaXxqPT.png
So here's what happens:
If the common bathroom light is ON first:
Turning the common bathroom fan from ON to OFF causes a 1-second flicker OFF in the common bathroom light.
Turning the master bathroom light from ON to OFF does NOT cause any flicker.
Turning the master bathroom fan from ON to OFF causes a 1-second flicker OFF in the common bathroom light.
Can you maybe guess what's happening here? What should I look for to find the issue?