I disagree. The CCP's modus operandi is self preservation through suppression of information. To those perpetrating the cover up what actually happened is irrelevant. The CCP is never transparent in matters that could make them look bad. From their perspective allowing an open international investigation would make them appear guilty regardless of the results because it would be unprecedented.

While I agree with you, I'm thinking that this is a response common to sensitive government institutions in general. If this lab leak happened in the US, would we suddenly invite CCP (or Russian/N.Korean/Iranian) agents into Fort Detrick to conduct their "investigations"? Which country would actually allow such a thing?

Consider for a moment the propaganda value of being able to say, "we visited their labs and witnessed first-hand their failure to {do some sanitization procedure} correctly! Oh how they clearly don't value human lives!"

My impression is that a thorough investigation is the normal thing for any major epidemic.

... so if an open investigation and free access would look better then a cover up then by your logic they would allow it leading to the conclusion reality looks worse?

No, the point is there is almost no way the party would sanction an external investigation, regardless of the actual situation.

I find this very surprising, but I suppose it could be true. Can you put some meat on the bones somehow? An anecdote or something?

Without going into subjective current event interpretation, the phenomenon is well illustrated in HBO's Chernobyl with the plant engineers and manager, or the show trial. Their discounting of material facts in favour of "the expected answer" by higher-ups is an endemic (and probably rational) strategy in response to ruthless (central plan+party politics)'d organisations.

I thought the lab leak hypothesis was pretty unlikely in early 2020 but having seen the how the Chinese state has acted since then it now seems entirely believable.

there were similar conspiracy way back in 2003 when SARS outbreak, except this time in 2019~, the social medias are way more viral than the virus and CCP apparently can't suppress these "contents/info/fakenews" even with the enormous help from WHO