I suppose there is always the chance you got some from a bad batch. Not unknown with any make of sable, but less likely with synthetics for the reason Einion gave.
Standard white spirit should never harm a brush as it contains oil anyway, so less wearing than IPA or thinners, as it does not dry out the hairs.
I personally use a drop or two of washing-up liquid in warm water after the white spirit.
Only point I would make is don't use hot water, it should only be tepid.
When you leave them to dry in the air, do you re-form the point on a soft cloth?

Ron

I use brush soap, but it sounds like shampoo is better. Is the brush soap too hard on brushes?

Have been thinking about the shampoo for cleaning. What sticks in my mind is that a lot of women complain that certain shampoos make their hair go 'frizzy' (sticks out/unmanageable).

Just thinking outside the box.

Ron

You can always go for a hairconditioner afterwards.

Marc

It can be. Some people swear by them but others have bad experiences, which initially led me to being suspicious that there was some variation in the soap itself that maybe accounted for the different opinions; but there are many variables. Bottom line with the soaps themselves, many are not much different from common hand soap* which of course are a heck of a lot cheaper!

*The Master's for example has a pH around the same as cheap motel soaps, so nothing special on that front.

Imagine the complaints if they cleaned their hair with soap

Anyway, as Marc mentions condition afterwards if you want to go the extra mile.

Einion

Master's is what I've been using. I actually don't use it too often, but it seems to do a good job of breaking up dried acrylic paint. I'll try using just regular soap/shampoo.

This is actually the thing to try to avoid having in the first place - prevention is better than cure.

I'd recommend setting up a double-bath rinsing system (two jars of water) and learning to use it religiously. Regular, thorough, rinsing makes a huge difference in how clean your brushes stay during use which greatly reduces the need to actually wash them, extending their useful life.

Einion