This will be a very very VERY rare thing to check for but get your head under the footwell with a flashlight to visually inspect your clutch pedal for any signs of wear or metal deformation at any of the hinge points where stress can be hard on those areas especially at the two points where the "helper" spring is attached. You want to see if anything looks amiss physically. I've had to deal with that once requiring a pedal replacement when I used a fairly stiff SouthBend Stage 3 clutch but I've only read of one or two other people having the issue.

Normally I would say that this issue should be related to a failing clutch master cylinder or a failing clutch slave cylinder or due old and worn out DOT3 fluid in the lines or air in the lines.