Why are LEDs in my home unaffected by voltage drop?

Your LED bulbs are most likely fitted with a wide-range power supply, operating from 100-240 Vac. Hence, when the voltage dropped, they were still within their operating limits and your LED bulbs were as bright as before.

Side note: You'd be surprised how many bulbs marked 220-240 Vac can actually operate down to 100 Vac or less. The nameplate numbers only tell you where it can operate, not where it can't.

You can get quite small but sophisticated LED constant current drivers that work across a universal AC power range. Here are a couple: -

Both work down to 85V AC.