Multiple Toilets Bubble when shower turned on
This is a classic symptom of a restriction/blockage of the main sewer line downstream of the toilets. No way to know for sure without scoping with a camera and/or running a mainline sewer cable.

You say


  We are in a HOA that maintains the water service (sewer)


It is the HOA's responsibility to clear this up. If you don't feel comfortable reporting it because you are subletting, have the leaseholder report it. How will you feel reporting it if there is a backup which floods the entire unit?