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"We discovered and patched this issue last month. However, a recent site update (unrelated to new Twitter) unknowingly resurfaced it. "

That's the second major bug in 24 hours that simple regression testing would have found.

What's the root cause of these issues? Did older test sets somehow get recovered or something like that?

It depends why it regressed. Even if the text processor library didn't regress, the dependency may have reverted to an older version somehow.