Search results empty possibly due to patch to 2.4.5-p9

Sorry – cross posting from Stackexchange but client getting antsy and I’m stuck.

Patched a site from 2.4.5-p8 to p9 last week. Now the search is not returning any results. The search box uses MagePlaza Advanced Search which shows results in a dropdown and that works, but the actual results page has nothing. (eg https://www.website.com/catalogsearch/result/?q=tea)

• Disabling that module changes nothing.
• We have re-indexed and cleared/flushed cache a million times.
• Also tested ElasticSearch which seems fine.
• Also checked in product attributes that Name and Sku are set up for search, resaved that too. (this worked when we had this error back in January 2023, also following a patch)
• Products are all set up for Catalog/Search.

Is there anything else we can test or try?

One thing is that at one point earlier in the week, a few days after the patch, SQL connection was lost for some reason and the whole site went down. When it came back there were no products in catalog pages until the ElasticSearch service was restarted – it had not restarted itself. The search issue was only spotted since then. No-one can say for sure whether the search was working before this ES issue happened, so we don’t know whether it’s a result of this or of the patch itself.

Thanks!