nostr-tools is somewhat erratic. Depending on the version it will take arrays or single-filters, and aggregate them or not in a single REQ or spread them into multiple REQs. It did this last thing for a while when we were investigating moving the entirety of Nostr to single-filter REQs, but that didn't work because the forces of ossification are too strong, so now on the latest nostr-tools you can get multiple filters on the same REQ but only if you use the same relay multiple times in a call to subscribeMap().