You could be looking for a two pole GFCI breaker.

It's also possible that it didn't work before with loads plugged into both sides of thw MWBC or whatever you've got here.

Thanks for your replys.
They both worked fine before, including the Press-To-Test function. They are lighting circuits that were always used at the same time.
The original breakers were single poles and wired with type NM cable.
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Is your three phase a delta or Y setup? Delta setup has some odd things happen with the neutral. You have to really know what you are doing. I don't don't know enough to be of more assistance than to to say you need an expert.

Thanks for your comments.
It's a standard Y configuration as most of the equipment is 120v. After a couple hours of tracing cables, we found the bootleg common that I suspected was there. I may be a lifetime DIYer, but I don't think I'm quite as clueless as the clown that did this one.

Mr.Clue, does this mean that you have purchased and installed the required two pole gfci breaker?

Not applicable.
What we had to do was to have the hot and neutral in the same cable rather than the hot in one 12/3 cable and the neutral 'bootlegged' in another 12/2 cable (where the black was not used).