A common issue we keep encountering on Magento 2.4 are intermittent blank category pages.
Magento product category pages often show blank.
It's become clear that the Magento 2 search indexes and category indexes are more intrinsically linked. In Magento 1 versions, the two areas were more isolated.
From the root folder, reindexing the following two indexes provided resolve in the short term:
php -d memory_limit=-1 bin/magento indexer:reindex catalog_category_product
php -d memory_limit=-1 bin/magento indexer:reindex catalog_product_flat
However, the problem would occur again.
Long Term Solution
Upon checking the indexes, one index was found to be set on 'Update on Save'. Presumably, this was forcing unecessary search reindexing and resulting in an incomplete reindex.
A better solution would be to set all indexes to 'Update by Schedule' inline with the Magento guidlines:
Make sure that these indexers are in “Update By Schedule” mode. If “Update On Save” mode is selected, some data can be lost if you make changes during full reindex.
Source: Magento Official User Guide
This problem may occur with incorrect PHP settings. Ensure your PHP master and local settings are inline with Magento release notes. The search index may fail to complete with incorrect settings.