Sydney Sweeney is at it again ... she just can't seem to help but flirt with Glen Powell online -- with his ass-baring magazine photo shoot very much still top of mind.
After flirtatiously claiming her costar's near-nude images were contributing to rising heat levels this winter, Syd showed off yet another personal glimpse of her hunky costar as he worked out onboard a yacht during their film shoot together ... which she threw up on IG.
Check it out .