Generally, you don't get hypoxic while using a clogged respirator. The problem in such cases is mild hypercapnia and increased work of breathing (Respiratory loading). Hypercapnia in general can cause short term cognitive issues but not long lasting issues. Respiratory loading can cause temporary pulmonary edema.

However, the respiratory loading and hypercapnia are extremely mild in the case of clogged respirators. Especially when compared to divers and snorkelers. The usual problem is that the respirator stops preventing contaminated air from reaching your lungs. As the filters become clogged, the air bypasses the seal around your face.

This is why I prefer a powered respirator. Zero work of breathing and positive pressure at the seals.