The bug is evidently related to visiting a materials trader. Once you do a trade, the caps can shrink to 100. It's not just a display thing. It actually caps at 100. So a relog after every visit to a mat trader would be in order.
Yeah I don't think it's solely linked to that, I didn't visit any mat traders yesterday (after being logged off all night), I dropped into a USS later in the evening (was mostly swapping ships and engineering Modules) and noticed the "Inventory Full" message when I went to grab something I had 138/100 of. The material vanished upon entering my cargo hold, so I didn't bother with the rest.
Checked my storage and everything was capped at 100.
I didn't use an SRV at all that day either.
Re-logging after I'd collected the things I could, did of course reset it, but it's definitely not linked solely to the mat trader. I can't tell you when it changed for me as I had the day off and was in ED pretty much all day, but it did nonetheless.
Hoping it's something they fix though at least it's 100 and not lower, so you're not losing out massively if you do happen upon rares at a USS (albeit massively annoying).