Actually.....your second time when the squats felt much lighter is because, your CNS has adapted itself to the weights of the 50KG thus you feel like the 30KG's extremely light.

When your workload has increased, you can do walkouts with your 1RM+30KG and stand for 5 seconds and rack it back, and try your 1RM immediately. It'll feel lighter. Eg, if your 1RM is 100KG+30KG and do a walkout.

And don't depend too much on the soreness as your gauge of performance, when you've gotten used to it, you won't feel any soreness anymore.