I've had the same problems. Seems worse on my largest playlist (13k) songs. Searching within the playlist takes several minutes, if ever, before it will locate searched songs. Also just crashes randomly when using this specific playlist. Very long delays also when adding a song to this playlist, between the time you choose to add a song and when text appears indicating that the song already exists in the playlist - or when adding a song and receiving notification badge that the song has been successfully added.
These issues don't seem as bad with smaller playlists. Like you, this issue began immediately after updating to iOS 18. I deleted and reinstalled Apple Music app to troubleshoot and there was no improvement. Hoped there would be a fix in 18.0.1 but there has been no change. To my knowledge, there has never been a limit on number of songs in a given playlist. Apple needs to issue a statement or give the caveat somewhere if they can't fix their issue, resulting in a per-playlist song limit.
I run a social media group that requires me to have access to this large playlist, so I'll be moving on from Apple Music if it can't be resolved soon. As an aside/rant: Apple Music has a few decent Apple-generated playlists, but is garbage with visibility/searching user-created playlists, compared with Spotify, etc, but I'd still stay with them if they can fix this issue. I'd rather support a company like Apple that pays artists (slightly) better, but their product also has to work. Does anyone know how to contact Apple support about this? Is the only solution via phone? Thx