I don't understand why objectivists seem to be held in low regard here. My exposure is limited to browsing a forum of objectivists[1] - they were indistinguishable from those here, though much more focussed on personal instrumental rationality in their topics.

I know they are formally a closed loop belief system limited by the writings of Ayn Rand (which I've not read), and have heard this belief system is flawed in some way. That sounds like a straw man.

I'm only interested in the steel man. What is the difference between rationality and objectivism?
The only one that comes to mind: Objectivism implies there is only one true way of some things, while rationality allows for individual variety in thinking processes (resulting from different information, experiences, terminal values, etc.)

However, if for one person their most desired thing is happiness - which can only be achieved through quasi-altruistic deeds - then I cannot see it as anything but objective and rational to carry out those deeds. Objectivism applied to the fulfilment of one's desires appears indistinguishable from rationality to me. Where am I wrong on this - or am I playing semantics?

[1] Knowing what to look for, I discovered the site again. They indeed are very skilled at applying instrumental rationality to various areas of their lives (exempli gratia what type of plastic surgery yields the most natural results?) - however in the Philosophy and Ethics sections, Ayn Rand philosophy abounds. They are describing the intent of an Important Figure (scary), without doing so for the purposes of then breaking it down; those that try the latter attack straw-man versions and are refuted.