How to generate a 555 fixed time delay even if trigger button is held on?
You are looking for an edge-triggered one-shot or monostable delay. The output should time out normally even if the button is held on.



Figure 1. Edge-triggered on delay. Add components (1) and (2).

Add components (1) and (2). A C value of 1 µF and R of 100 kΩ will allow it to rearm in 0.1 s.

How it works


At rest both sides of C2 (the new capacitor) are charged to battery + by R1 and the new resistor.
When SW1 is pressed the left side of C2 is pulled low and the capacitor action pulls the right side low too. This triggers the 555.
Whether or not SW1 is held, the right side of C2 is pulled high by the new resistor allowing the timing cycle to begin.