Shields hit with ion damage prevent skill increases.

[Shimshai]

In this specific incident, my shield was being hit by an ion stunner. While the first shield is locked down by ion damage, any additional conventional shield strikes don't result in the crewman gaining a shield skill increase. I haven't tested this to see if it's always the case with all types of ion weapons, just something I noticed and wanted to mention it here.

It may even be working as intended. I will update if I get any more specific information.

[Dirka]

Ion damage stops shields from being manned. If you look closely, you can see your guy stepping away from the console and returning to his idle animation.

[Shimshai]

Oh well that sounds exactly like intended behaviour, so disregard this bug report