OK, the team needs some help with clear defined steps to reproduce this infuriating bug. The fact it happens is clear enough, and consequences are disastrous. However, it always happens when not expected, and so far nobody can tell the way to force it to happen. It's been three years! Please – if anyone knows the exact steps – give them here in the comments.
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Same, artboard numbers are 1 2 4 5 5 7 7 8 10 10 and after 10 the numbering returns to normal. Creating a new file doesn't help, same issue occurs.