It depends on the type of material your work uniforms are made from. If they are 100% heavy cotton, the flame proofing will be in the form of a chemical solution dip, (usually a phosphate), which has to be re-applied periodically as it slowly gets washed out of the cloth- but they're not dipped on every wash. This material is old technology- cotton overalls shrink so much when laundered, plus quality control of the effective proofing is difficult to assess. Proofed cotton has been largely superseded by polyamide-mix materials, the market leader is 'Proban' in the UK.
Either way, 'grit' in the pockets is not normal or acceptable- they're not clean, are they?- and IMO the laundry quality-control is at fault.
Wood work but can't!