Of course every round lossy of encoding further discards data. RemoveData(RemoveData(source)) is always going to look worse than just RemoveData(source). Newer encoders manage to remove less visual data per byte of storage used but there’s no way re-encoding is going to ever look better.
If I understand it right, some lossy codecs can be implemented in an idempotent (and still standard-compliant) way, so there would be no generational loss (in select specific cases, e.g. matched settings). I'm also aware that e.g. JPEG-XL can reencode JPEGs without generational loss, while still improving compression efficiency a bit. But I never looked too deep into the math.