It is a problem but something out of my control and up to relay operators. It is likely they are deleted faster as they are encrypted and from throwaway keys so relay operators have no way to determine that the nostr events are important of if it is just junk spam. If you want to be certain your nostr events are kept by relays then the only way to be sure of this is to run your own. By default the posts are sent to nos.lol primal.net and offchain.pub. You can see what relays your posts are sent to by checking the Dev console after making a post.

Could you update Nowhere so that it puts a client tag in each event in the future?