A signal does not exist, but it does exist when using a signal tap to capture it.

i) As mentioned by sstrell, it is possible that there are timing issues in your design that are affecting the functionality of the startup signal. Adding the signal to SignalTap might be introducing additional delays that help correct these timing issues temporarily. You might need to review your timing constraints and ensure that all paths are properly constrained.

ii) Try to run a simulation to ensure that the design function correctly.

iii) Like what FvM stated, if the startup signal is related to system reset or initialization, ensure that it's synchronized properly across different clock domains. Un-synchronized resets can cause unpredictable behavior during startup.

Regards,

Richard Tan