It's probably because this Primal relay, since it's a free relay it's an "open gate" for spammers.
I'm using only these wot relays provided by nostr:npub1utx00neqgqln72j22kej3ux7803c2k986henvvha4thuwfkper4s7r50e8 + 2 paid non-wot relays + mostr.pub (to receive Mastodon posts) and almost all spammers vanished