I'm wondering whether it's air introduced during mixing or gas generated during curing due to moisture contamination. If it's the latter case, could someone recommended a moisture scavenger or some other solution?

Is the silicone mould new or has it already made a few parts? - Also, do you wash the mould out (soapy water for example) before use? - If you do, this would be a prime candidate for a problem. It's strange that you had the same problem with the previous FastCast too; that resin has high levels of moisture scavenger in which made it very insusceptible to moisture reactions so I'm thinking there might be an issue with the process somehow.