It looks like you are finishing too close to the seam too soon in the process. When you do the heating subsequent to soldering the bezel onto the back plate, if you have filed the bezel seam to its final location, the solder will recede from the edge.
1. Use medium solder to secure bezel to back plate. Flux plate with paste flux, then set bezel in place and gently move around to get bezel in true contact with backing. Place several pieces of clipped round solder inside the bezel and touching both bezel and plate. Heat the plate quickly, perhaps tipping the piece so you can apply heat from the underside. When some of the solder flows, complete the operation by circling the bezel, just outside the perimeter. While it is still hot, use your solder pick to check that the surface tension has caused a complete run, both inside and outside.
2. Soldering the bezel down is more successful if you have raised the piece from your soldering block whether charcoal or fire brick or... My most successful material for raising the plate is titanium strips, bent so they are on edge - an "L" shape works. Don't use the elevated wire screen, it is a heat sink. You can make a nest of binding wire or any number of other materials. The titanium works best because it is not a heat sink and you cannot accidentally solder to it. Ready cut strips are available directly from Knew Concepts or from Rio Grande. They also make good holding clamps.
3. Now trim the piece, but do not do a final sanding. Leave the cut edge almost flush with the bezel. It should be a right angle edge..Add your decorations to the piece, the solder for them will flow nicely at the vertical edge.
4. Only when you have completed all heating steps, should you do the final edge finishing. Make sure to keep the edge very nearly square, if you round it like your picture shows, you can sand or polish right thru the seam.
5. Using a heavier back plate and a heavier bezel might be your solution.
6. To demonstrate the receding solder problem - take a small bit of silver plate and solder a piece of bezel to it. Then file and sand the plate to your desired finish. Heat it again and note that the solder line has receded.