The Latin Grammy-winning singer sparked some unfortunate chatter after rocking a tight pair of leather pants for his performance at the La Onda Music Festival in Napa Valley over the weekend -- and it all has to do with a noticeable imprint around his crotch area.

AF thrusts his hips forward while jamming on stage -- giving fans an unexpected glimpse at the outline of his junk ... and like we said, some don't seem to be that impressed.

Of course, it didn't take long for the bulge to go viral ... with over 2 million people tuning into the TikTok and over 15,000 people sounding off in the comments. While some fans joked about being scarred by the footage, others playfully poked fun, joking "he can't have it all."

Another fan chimed in with ... "Napa valley at winery.. where you find tiny grapes 🍇."

While Alejandro has yet to address this burning hot moment -- he DID upload a photo from his show ... where his bulge again took center stage. He wrote ... "Huge thanks to my cool people of Napa for such a fun afternoon! Til next time California peeps!"

So, maybe he's signaling an "IDGAF" message of sorts -- and if so, good for him!

Fans were far more generous in the comments of this upload, praising the Mexican music legend for his memorable performance ... BTW, this dude is an absolute legend in the Spanish music scene, so he definitely gets some goodwill from his loyal fans.