The liblognorm "rest" parser was introduced some time ago, to handle cases where someone just wants to parse a partial message and keep all the "rest of it" into another field. I never was a big fan of this type of parser, but I accepted it because so many people asked. Practice, however, showed that my concerns were right: the "rest" parser has a very broad match and those that used it often got very surprising results.
A key cause of this issue was that the rest parser had the same priority as other parsers, and most importantly a higher priority than a simple character match. so it was actually impossible to match some constant text that was at the same location than the "rest" parser.
I have now changed this so that the rest parser is always called last, if no other thing matches - neither any parser nor any constant text. This will make it work much more like you expect. Still, I caution against using this parser as it continues to provide a very broad match.
Note that the way I have implemented this is not totally clean from a software engineering point of view, but very solid. A cleaner solution will occur during the scheduled rewrite of the algorithm (later in spring/summer).
Note that existing rulebases using "rest" may behave differently with the new algorithm. However, previously the result was more or less random, so any other change to the rulebase could also have caused different behaviour. So this is no compatibility break as there really is no compatibility to retain.
This will be released with 1.1.2, probably in early may. If you need it urgently, you can use a daily build.