Front left sti shifter smaller lever moves with main lever during shifting

Hello All

For some reason, my front left STI shifter (Shimano Tiagra Triple), when I try to upshift by pushing the smaller lever, the main lever also moves with it. Not only that, sometimes they both just stay engaged instead of returning back down. What is the best way to see if this can be fixed?

I don't mind replacing just the front left shifter, and I kind of want to run Shimano 105 Front left triple shifters (since both FD and RD are all Shimano 105), but does this work? Can I have the front shifters using Shimano 105 and the rear gears shifter using Shimano Tiagra?

Thanks for reading.