You are right it’s not in the RFC but it’s a standard.
The RFC does not precise how to handle line feed and does not precise how to end a MSG.
Rsyslog’s doc says:
Industry-strandard plain text tcp syslog uses the LF to delimit syslog frames. However, some users brought up the case that it may be useful to define a different delimiter and totally disable LF as a delimiter (the use case named were multi-line messages). This mode is non-standard and will probably come with a lot of problems. However, as there is need for it and it is relatively easy to support, we do so. Be sure to turn this setting to “on” only if you exactly know what you are doing. You may run into all sorts of troubles, so be prepared to wrangle with that!
You need to configure Rsyslog to send a one-line log to Graylog.
What’s your Rsyslog input ? file ? network ? Can you share your configuration ?