I have a little issue with my Apple Watch Ultra.

I run regularly and since winter came I grew a little sick of having my watch stuck to my arm during a workout to track my HR, because that spot gets a bit cold. To fight this, I bought myself an HR strap, and decided to cover my arm, and get my watch on top of my running jacket (so the sensor does not have contact with my skin). HR gets offloaded to strap nicely, but because my watch does not touch skin it locks itself out. Has anyone had a similar issue before? How to get my watch unlocked during a whole workout? I use the WorkOPutDoors app as well.