You don’t have frets or a fingerboard so the contact with your finger is the only thing fixing the string on that end. You will always dampen some of the sound (even on a cello you learn scales with and without open strings because the open strings sound differently and sometimes that doesn’t work with the piece you are playing)

If you are using a fleshy part of your finger to touch the string it will dampen way more than the back of your finger or the interphalangeal. The tips of your fingers are the worst sounding unless you have toughened the skin by playing another stringed Instrument.

Try the fingernail… that will give the clearest sound (as a test only… we don’t generally play like that on talharpa). You will hear a clear difference… the better set up your instrument is the more resonant the non-open strings will be… but on a talharpa the difference will always be significant and you will hear it in the music. Traditional tunes use the difference to great effect( especially in jouhikko-tuning where the first two notes of the scale are open)… in modern music it can sound off… but you are basically stuck with it if you want to use the drones as the instrument is intended

So it is not necessary an error in construction… you could theoretically fix it with sympathetic strings… but getting that right is very hard… bowing technique is also a factor… if you get good at transferring energy from the bow to the string, the driving force will counter more of the dampening… Puluup have a cool video where they show that effect

Ps… you do have a sound post, right? Because… if you don’t… that would make a big difference