Display of the triangles representing the flows in the model diagram

On my computer, the triangles representing the flows in the model diagram are not displayed.

I don't know where this is coming from, because if I draw these signs directly into R, it works.

tri1 <- c(-0x25BA, -0x25BC, -0x25C4, -0x25B2)
plot(seq_along(tri1), pch = tri1)

tri2 <- -as.hexmode(c("25BA", "25BC", "25C4", "25B2"))
plot(seq_along(tri2), pch = tri2)

On my computer, with the Linux OS (see details below), I tested the interface with two browsers (Firefox and Chromium). The triangles are not displayed.

The sunshine website runs on a Linux server and the triangles are displayed.

On my computer with the Windows OS and the Firefox browser, the triangles are displayed.

> Sys.info()
                                     sysname 
                                     "Linux" 
                                     release 
                         "4.15.0-96-generic" 
                                     version 
"#97-Ubuntu SMP Wed Apr 1 03:25:46 UTC 2020" 
                                    nodename 
                                  "ANPI1780" 
                                     machine 
                                    "x86_64" 
                                       login 
                                    "irstea" 
                                        user 
                                    "irstea" 
                              effective_user 
                                    "irstea"

> Sys.getlocale()
[1] "LC_CTYPE=fr_FR.UTF-8;LC_NUMERIC=C;LC_TIME=fr_FR.UTF-8;LC_COLLATE=fr_FR.UTF-8;LC_MONETARY=fr_FR.UTF-8;LC_MESSAGES=fr_FR.UTF-8;LC_PAPER=fr_FR.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=fr_FR.UTF-8;LC_IDENTIFICATION=C"
>