Zend certified PHP/Magento developer

Product pages loading partially occasionally, without any obvious errors

I am facing the following issue on an already-live site on Magento 2.3.2:

product pages occasionally don’t load completely, missing (amongst other) product attributes / swatches (clothe sizes in our case), which are mandatory and thus preventing orders to be placed.

This happens:
* about 70% of the time the first time a new browser visits a product page
* about 40% on each refresh

I have tried switching magento modes, flushing all caches multiple times, changing PHP version (currently on 7.2 served as FPM app through Apache). As it is already live I have not yet tried to start disabling add on modules, although I see none that could be directly affecting product pages.

No obvious errors in JS console, Magento backend, or Apache logs except:
AH01070, which relates to error parsing script headers / premature end of headers, which I’m currently looking into.

Any one encountered anything similar? If not, any ideas how to rule if it’s something Magento-related or webserver misconfiguration?

Thanks

(not adding many magento-related tags until I’m certain)