Interesting problem. For me to be able to make an educated guess, it'd be great if you could list the exact modules at work here. Every filter, every envelope is unique in its design and might respond differently to the same input. Here are some questions for clarification and guesses from me:
Both are being triggered by the same gate source.
By both, you mean the envelope and what else? Are you applying the gate signal to the filter in addition to the envelope? Or are you syncing/resetting the oscillator with the gate signal?
The envelope is adsr and the attack is at zero.
What exact model is the envelope module? Attack at zero does not necessarily mean an "attack time" of 0 seconds. Usually, the attack stage takes at least a few ms to reach full amplitude. This might be an issue.
There's nothing like a CV amount or attenuator or envelope amount knob on either the filter or eg.
Next potential problem: The envelope output amplitude might be too low from the start. If it's only putting out up to +5V, that might still not be enough to open up the filter as much as you'd like it. Check the manual, it ought to list the envelope output amplitude under the specs.