Normally, when wearing a mesh body (or part-body), you also wear an Alpha layer to hide the (part(s) of) the legacy avatar shape (and therefore the corresponding legacy clothing layers)...
In order to try and reproduce your issue, I however wore a mesh body and removed all Apha layers it was associated with (thus getting old shape "poking" through the mesh body)...
Not sure what you mean by "Apply top and bottom clothing layers"... I wore legacy clothing items (top and bottom), and did not see anything wrong...
What HUD ?
You don't need a HUD to wear items ! And a HUD would be incapable of toggling the viewer alpha masking on/off (toggles are in menu Advanced -> Rendering) !
I'm not seeing any render glitch. My guess is that you are using some commercial mesh stuff that abuses old render glitches (since fixed) to achieve a non-standard result... Or you are seeing a driver/GPU specific render glitch.
Without any detail on what particular mesh you are using, what is your hardware and what are your OS and driver versions, and without viewer log, I'm unable to diagnose your issue. Please, fill-in a proper and full bug report, abiding to the letter to
the rules.
Yet, there has been no change in rendering from v1.26.20.27 to v1.26.20.35 included. Did you test v1.26.20.35 and does it change anything for you ?