Sorry for not checking the log first--the answer is indeed there. The search may be assuming that the track title is an album title.
I use the plugin almost exclusively when listening to radio streams, so that when the stream metadata sent includes band name and track title I can see the lyrics (if found) for the playing song. Stream metadata almost NEVER includes the album title, just artist and track title. This works perfectly fine for all the other sources the plugin retrieves, just not this one. An example from the log for the song "Starlight" by the band "Waterland":
09/08/2022 03:26:39 [DEBUG] Lyrics request: Waterland - Starlight - - Metal Archives
09/08/2022 03:26:39 [INFO] Metal Archives tries to load the lyrics...
09/08/2022 03:26:39 [DEBUG] The constructed identity URL: https://www.metal-archives.com/albums/Waterland/
09/08/2022 03:26:39 [DEBUG] gzip compression detected
09/08/2022 03:26:39 [WARN] The identity pattern Starlight\n</td>.*?lyricsButton(?<identity>.*?)" didn't match!
09/08/2022 03:26:39 [INFO] No lyrics found.
09/08/2022 03:26:39 [FAIL] no lyrics found from Metal Archives
If I'm interpreting this right, it's looking for a list of albums from the band Waterland, and then trying to find the title "Starlight" therein which fails because it's a track title not an album title.
I'm not surprised this site is a tough nut to crack. If it helps at all, there is a source plugin for Foobar's "lyric show panel" just for this site that still works fine after 4 years. If looking at how it works might help (it's also a DLL so opaque to me), it's downloadable at https://hydrogenaud.io/index.php?PHPSESSID=olto6kq35b21uv73jjal04vumh&action=dlattach;topic=108084.0;attach=11999