Lower leaves on Paphiopedilum turning yellow

[Sirinne]

Hi guys!
I have had Phals for a while and we seem to get along, but I got a new Paph Venustum orchid for about 3 weeks and its lower leaves are turning yellow.

I have already lost one leaf entirely over the course of about 10 days, now the second one is feeling soft to the touch and turning yellow, took about a week to get to the state it's in. I think all the other leaves might be a bit less crisp to the touch.

It's planted entirely in bark, repotted after purchase. I water with lukewarm tap water when the upper half of the substrate looks dry and the lower half looks somewhat moist, once or twice a week. Run it for about a minute under the tap. The fertilizer is slow release orchid fertilizer, I don't add any with watering. Light is north window, no direct sunlight ever. Temperature in the room is around 22-24 C.

My guess is I am probably watering it wrong, but I don't know if I might be giving too much or too little. Could you guys give me any advice what the problem might be?