This image is created by complex image reconstruction algorithms that propose alternative solutions which are averaged. Some of these alternatives do not show a ring structure but instead only or mostly the three knots or clusters. The image in the question is an average of all those options.
In the article "First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole" this is discussed in the section "7.5. Is Sgr A a Ring?"*. The authors give three potential causes 1 It's not a ring structure 2 The object is truly a ring but the image is distorted due to scattering 3 The object is truly a ring structure but distorted due to imperfect observations (there's an inverse Fourier transform involved with an incomplete spectrum/uv-plane).
The authors believe that option 3 is most likely because they get the same effect in other objects that are known to have a ring structure.
It is also discussed here: https://skyandtelescope.org/astronomy-news/astronomers-unveil-image-of-the-milky-ways-central-black-hole/
What the reconstructed images don’t agree on is the bright knots dotting the ring. Knots are natural, due to the tangled magnetic fields threading a black hole’s tutu of hot gas. But the knots in the Sgr A* images move depending on which reconstruction you use, and they tend to line up along the directions with more telescopes, Özel warned. “We don’t trust the knots that much,” she said.
There are also simulations of images that show multiple bright sources due to gravitational lensing. When the ring is seen from the side then we see the ring in front of the black hole plus two lensed images from the ring behind the black hole. See for instance this simulation by Jeremy Schnittman from NASA.
Or the very old simulation from Jean Pierre Luminet
Image from Wikipedia copied from the original source Astronomy and Astrophysics, vol. 75, no. 1-2, May 1979, p. 228-235
Here you see a warped primary image where the bright spot is due to a Doppler effect. In the black center is the shadow of the black hole. Around it is a thin disk of light which is a secondary image from light that has traveled a path of 180° around the black hole.
The bright spots also show up in simulations which simulate inhomogeneities in the gas that is falling into the black hole along warped stream lines. Wavy pattern may occur due magnetic fields and due to the rotation of the black hole.
An example simulation is by Dexter and Fragile in arXiv:1204.4454 "Tilted black hole accretion disc models of Sagittarius A*: time-variable millimetre to near-infrared emission", where standing waves are caused by a warping due to Lense Thirring precession in the tilted alignment of the black hole rotation axis and the accretion disk causing warping
But whatever we think or believe it is, it is speculation because we do not even know whether the spots are imaging artifacts.