I don’t understand when all lights flicker after adding second light switch

[Jatsip]

Adding an image of my set up

Basically this is the run across like 60-75 ft. The main line comes into a switch to control the entire line. That continues through 8 lights and then into an outlet. Originally After that outlet I had a light but I wanted to be able to control it independently from the other lights. So I added a 2nd light switch just to control the last light. The breaks out from the outlet.

So after completing the work everything works and turns on. But when I switch the 2nd light on or off, all of the subsequent lights will flash on and off quickly. I’m not really sure why and was hoping someone could help me out. Thanks

 

[telectrix]

sudden volt drop when single light is switched on

 

[littlespark]

I don’t see much wrong here… 75ft is what? 25m in metric. Not too long.

Looking at the picture, The wire I highlighted in yellow doesn’t need to there. Unless your code states there has to be a neutral at switches, even if unused, you could lose at least one of them.
And the hot I marked in red should be going to the switch output, not the common hot.

Other than that, the only thing I can think of is somehow the new light is in series with the 8, and the lamps are LED…That could cause flashing

 

[Jatsip]

I don’t see much wrong here… 75ft is what? 25m in metric. Not too long.

Looking at the picture, The wire I highlighted in yellow doesn’t need to there. Unless your code states there has to be a neutral at switches, even if unused, you could lose at least one of them.
And the hot I marked in red should be going to the switch output, not the common hot. View attachment 90606

Other than that, the only thing I can think of is somehow the new light is in series with the 8, and the lamps are LED…That could cause flashing

I actually did what you are suggesting but didn’t draw it properly