How different is this to gitlab’s open core model?

That's a really good question that I don't immediately have a satisfying answer to.

There are some differences I can point out though:

• Gitlab has demonstrated its commitment to keep the core of their product, though limited in features, free and open source. As of now, BW's clients cannot even be compiled without the proprietary SDK anymore.
• Gitlab was always a permissive license (MIT) and never attempted to subvert its original license terms
• Gitlab-EE's "closed" core is actually quite open (go read the source code) but still squarely in the proprietary camp because it requires you to have a valid subscription to exercise your freedoms.

Is this a permanent change?

It'd be quite trivial for them to do in technical terms: Either license the SDK as GPL or stop using it in the clients.

I don't see a reason for them to roll it back though. This was decided long ago and they explicitly decided to stray away from the status quo and make it closed source.

The only thing I could see making them revert this would be public pressure. If they lose a sufficient amount of subscribers over this, that might make them reconsider. Honestly though by that time, the cat's out of the bag and all the public goodwill and trust is gone.
It's honestly a bafflingly bad decision from even just a business perspective. I predict they'll lose at least 20% but likely 30-50% of their subscribers to this.

Is the involvement of investors the root of this?

I find that likely. If it stinks, it's usually something stinky's fault.

Are we overreacting as it doesn’t meet our strict definition of foss?

They are attempting to subvert one of the FOSS licenses held in the highest regard. You cannot really be much more anti than this.

An "honest" switch to completely proprietary licenses with a public announcement months prior would have been easier to accept.