In my project I also use the Ticker.h but asfaik this uses os_timer instead of hw_timer.
Before I wasn't using the Ticker library, and the dimmed bulb was not flickerering, I don't know for sure if this is causing the problem because I changed a lot more in my code, but nothing with timers.
I searched through the esp8266 arduino library but I can't find any code which is using timer1 except for Tone.cpp core_esp8266_waveform.cpp. I also red that PWM is using timer1. But none of these am I using in my code.
How can I find out why the timerISR is fired, can it be caused by the Ticker library?