Yeah this does seem strange. May want to shove a snake down it before disassembling.
If your air admittance valve isn't working (plugged? Usually they fail they other way) I could see it causing a vacuum if it isn't letting air in.
I recently replaced both of ours (kitchen sink and bathroom sink) and found the ones at Home Depot are screw in replacements (even shorter so they are easier to put in).
https://www.homedepot.com/p/Oatey-Su...9230/205395535
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