I've tried following a 'default cube' tutorial on Youtube. which is a glowing sphere inside a cube that has a cloth sim on it. I've used the same settings. When viewing it in the viewports the spare collides with the cloth object moving it around. But when I try to render it, as a still frame or animation, parts of the cloth disappear and the sphere is outside this mesh. I have tried it also with a default principled BSDF incase the transparent shader was causing the issue, but the effect is the same
here's a link to the tutorial
and a google drive link to the blend file and a screen shot showing the difference between the rendered image and the viewport.
I am using the latest 'official' release o blender (not a beta or alpha) and I'm running it on a mid 2015 iMac with Big Sur OS
Hope someone can help