My muffin tin has separate pieces to make the pockets, but I have had no problem with them de-soldering - maybe they are welded on?
But this made me think of another tip: don't use tin-plated muffin tins. The lead will alloy to the tin and will not
come out of the mold. You'll have to cut it out, destroying the muffin pan.
The guy who taught me how to cast showed me a tin like this with the ingots permanently in place. I was able to teach him what caused it because I had read about it on the web before my training session.