I just came across this issue after using the flogger node in "Select Object mode" then switching back to "Full Message Object". The "red triangle of doom" appears and will (sometimes) go away, and sometimes not. It's random.
Running latest everything on Raspbian. While no big deal to fix (delete node and start again), maybe the author? may wish to take a look.
It would probably help to be explicit about which node you are actually using as there are 3 listed on flows.nodered.org
Though given none of the 3 have been updated in over 6.5 years you may be out of luck with any of the authors being interested in fixing things. 2 of them are forks of the original.
That is a bug as it should ignore the msg field when Full Message is selected.
Normally one would post an issue on the node's github page, but there is no github page linked from the node. I, generally, would not use a node that does not have a public repository.
It isn't a missing default, the problem arises if you enter an invalid value in the msg field in Select Object mode, then switch to Full Message mode. The msg field is no longer visible so there is nothing to indicate where the problem is.
How am I supposed to fix it anyway when there is no link to the source repository?