Diagnostics?

I've been fiddling with multi-line comments and the bounding
characters. Naturally-paired characters  e.g. #`(...)  #`[...] #`{...}
all work well, but with other boundary characters like #`@....@ or
#`!....! produce odd, displaced, diagnostic messages. Reproducing them
is so easy, I'll leave it as "an exercise for the reader".

If unpaired boundary characters like these are legal, presumably
there's a problem with the parser or compiler?
If they aren't, maybe a) the docs should be more explicit, and b), the
diagnostics should say so, immediately?

perl6 -v
This is Rakudo Star version 2019.03.1 built on MoarVM version 2019.03