It's a known persisting glitch from the previous version, there's a few bugs from previous build that haven't been solved yet, and some that were solved, or thought to be, and have managed to weasel back in other ways
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As the previous version had it suggested as well - I recommend organizing your shop to house tool-heads/parts and grips ready to be finished rather than pre-assembled so that you do not run into this issue of item's "breaking"
Do not. However.
Construct a Wood Shield part and keep it on hand.
On Reload - Shields not only break like items
but the wood part tends to become solid, and once that happens, it is useless - May not always happen, but won't ever happen if you wait to do the wood part last