Can hypoxia cause brain to bleed / or cause brain injury if left untreated?

Bleed not likely but injury yes. Indeed when the brain gets no oxygen it begins to be injured/die in seconds.

The effect of significant hypoxia runs from rapid death to damage to sensitive brain areas, to transient loss of consciousness with full recovery. Bleeding is not usually associated, and if so, there may be a primary disorder causing secondary hypoxia. In most cases hospitalization with intensive care is required.