Not knowing the rest of your config - or what your other events look like, I'd still guess that the regex is a bit too greedy. I have not experimented with the PREAMBLE_REGEX setting, but I guess that this might actually happen before linemerging(?), or is it perhaps a special purpose TRANSFORM à la nullQueue. Somebody more knowledgeable may have that answer.
If you change your regex to the following, it should at least not be too greedy.
Thanks a bunch. At first I thought the regex you provided didn't work at all (but I was just being stupid). It still doesn't work for PREAMBLE_REGEX but I discovered I was being to greedy so maybe I can use your answer in transforms.conf. Thanks again!