it actually already works lol, the lrc never have 'length' attribute so it was never the culprit I think,
Apparently it takes quite a lot while before Poweramp can detect the lyrics, I rescan the lyric, the library, and even restarted the app but nothing. I leave it alone for a couple of hours, come back, and the lyrics now show up, no idea what's wrong lol
though when the lyric is embedded inside the mp3 file itself by writing the lyrics directly in its tag (I'm using AutomaTag) it's instantly recognized.
so.. yeah, it actually works but for some reason Poweramp just need time to detect the lyrics, this is only for .lrc files, if I edit the lyric tag itself, Poweramp can show it right away, if the lyric has timestamps, then it'll show up as synced lyrics.
Edit : correction, rescanning the whole library (All Songs > Three Dots > Rescan) make Poweramp show the lyrics immediately, apparently I just rescan in wrong category (sorry for that).
thank you