Compound model monsters, like in Malice, often the upperbody/subordinate part missing), this bug doesn't happen in WinQuake 

You might want to file these bugs or check and see what the status is, if they have already been reported.

https://sourceforge.net/p/quakespasm/bugs/

https://github.com/sezero/quakespasm/issues 

@ dumptruck_ds I don't see this issue listed there?

also I've narrowed it down

something is wrong with QS's MOVETYPE_PUSH, MOVETYPE_NOCLIP, MOVETYPE_STEP... because when used for body compound mdl monsters, the attached body is either invisible or not updating fast enough - so looks jittery (like I mentioned these bugs don't happen in winquake) 

I don't have malice or whatever, therefore I don't know what it is about. If someone has a well-tested patch, that would be welcome of course. 

airquake tanks also have a similar problem... try it yourself; look at the enemy tanks' turrets when they drive around

https://www.moddb.com/mods/air-quake/downloads/airquake-103 

Do ironwail and/or vkquake (or any other source ports) have this issue? 

Do ironwail and/or vkquake (or any other source ports) have this issue?
yup

I think joequake or some port with the old quake underwater effect didn't have this issue 

OK, created a ticket for this at https://github.com/sezero/quakespasm/issues/118

(For future reference, I don't follow this thread daily, I just happened to notice it by chance: Please use the issue trackers that dumptruck_ds noted above, like everyone else, otherwise bug reports here will easily get forgotten.)