This has been observed to occur with smaller stacks of imperial guards

Can confirm this also happens with Imperial Guard (larger stacks as well). 𝕭 was not involved.

It occurred during an attack on the planet's defenses after the siege had already landed, and caused 180,000 of 200,000 brigades to switch sides. I suspect attacking the planet under siege had something to do with it.

I've managed to make this bug reproducable! Attacking a world (to destroy ships & defenses) that I already had under siege caused 100,000 out of 190,000 brigades of Imperial Guardsmen to defect to the other player's side!

To reproduce this bug:

a)Start a siege of an enemy planet that's still building ships or defenses.

b)Before the siege starts, bring a fleet to the planet and attack it. Select 'destroy ships and defenses'.

c)Watch are your armed forces decide they're much rather fight for the other guy!

Thanks, this is indeed reproducible. Should be tagged a priority fix for Era 4 release.

New update on the defection bug: third parties can trigger defections too! Bugzzz was in the midst of putting someone else to siege when Imperator attacked his fleet from orbit, causing Bugzzz infantry to defect to the planets owner (not Imperator).

Also, forgot to mention: For some strange reason, while this bug is occurring you can see transports taking off and landing on the planet under siege. The transports themselves don't defect, but for some reason they take off and land. They're not carrying infantry while they're doing this (that I can see) and don't show up as a fleet you can select when zoomed out beyond the planet-scale: from all appearances, they're docked at the planet... until you zoom in and notice they're not.

@mdfification: Thanks for the repro steps—definitely helpful!