Yes.
If you stand back and notice at how much energy is being used to produce those new canes and also to provide a fruit load while also defending itself… you will see that the plant needs more nutrition and water.
Alot of folks are posting similar results on social medias… and alot of folks give up on them due to their demands.
Their root sytems are more shallow than blackberries which go deep. Blackberries also have thicker roots and wander deep and far to protect itself against lack of nutrients.
Hand fluff the area around the plant to ensure that whatever you are using as mulch isnt compacted… then add a dressing of fresh wood chips or compost… add a handful of a good fertilizer like 13-13-13 with sulfur or two handfuls of a lesser fertilizer like Holly Tone with sulfur and water diligently while in the early stages of fruiting. then pull back on the watering a week before peak ripeness.
Also going forward if you want a healthier plant do not remove spent canes until after winter if you live in a colder climate… they are not fully spent until after winter.
Your note about performance within 2 yrs of planting and decline after a good crop suggests that you need more nutrition and more water and better root development that will occur during the late fall. If you insist on removing ‘spent canes’ after harvest then you will need to top dress heavier with wood chips or good mulch to supplement the loss of its secondary food source.
I cannot rule out other diseases nor insects… but with better nutrition and water and a porous top dressing that holds moisture as well as drains well into the roots i think you will see an improvement on health.
Rasps will sacrifice their own tips to ensure crown survival… and will force laterals when conditions improve… such is their nature.