555 What What What ...

No thats normal, and i already did not liked the first version ;-) why? The pot is at a wrong and also critical point in circuit. This pot is a current to voltage transformer, as any resistor ;-) , and it lifts or drops the voltage for 555 thresholds !!! This depends on the current into and from cap, but the exact 1/3 and 2/3 thresholds are gone with this place of tuning pot! Measure pin 2+6 it suppose to be triangle/sawtooth. But if you increase resistance additional to the triangle form, you can see jumps up and down which lifts/shifts the thresholds and lets 555 work faster. Move the pot to the upper resistors and everything will be fine. Hope this helps.

Usually two diode s are used as well. But I have tried many many 555 pwm circuits, and non provided fixed frequency, I came to conclusion, Not to use 555 for pwm purpose. I think the commercial companies have tried to produce a general purpose chip,, and they have,,,, it is only good for timer circuits, and nothing else. . However much they try to push sales up.... By claiming it can do everything,,, like 741.